diff --git a/app/assets/javascripts/web-dev-listener.js b/app/assets/javascripts/web-dev-listener.js
index a1802604b..dc0e5f67d 100644
--- a/app/assets/javascripts/web-dev-listener.js
+++ b/app/assets/javascripts/web-dev-listener.js
@@ -1,5 +1,11 @@
 // TODO: don't serve this script from codecombat.com; serve it from a harmless extra domain we don't have yet.
 
+var lastSource = null;
+var lastOrigin = null;
+window.onerror = function(message, url, line, column, error){
+  console.log("User script error on line " + line + ", column " + column + ": ", error);
+  lastSource.postMessage({ type: 'error', message: message, url: url, line: line, column: column }, lastOrigin);
+}
 window.addEventListener('message', receiveMessage, false);
 
 var concreteDom;
@@ -30,8 +36,9 @@ function receiveMessage(event) {
         console.log('Ignoring message from bad origin:', origin);
         return;
     }
+    lastOrigin = origin;
     var data = event.data;
-    var source = event.source;
+    var source = lastSource = event.source;
     switch (data.type) {
     case 'create':
         create(_.pick(data, 'dom', 'styles', 'scripts'));
@@ -79,12 +86,8 @@ function replaceNodes(selector, newNodes){
     $newNodes.attr('for', firstNode.attr('for'));
     
     newFirstNode = $newNodes[0];
-    try {
-      firstNode.replaceWith(newFirstNode); // Removes newFirstNode from its array (!!)
-    } catch (e) {
-      console.log('Failed to update some nodes:', e);
-    }
-    
+    firstNode.replaceWith(newFirstNode); // Removes newFirstNode from its array (!!)
+
     $(newFirstNode).after($newNodes);
 }
 
diff --git a/app/core/initialize.coffee b/app/core/initialize.coffee
index b81841a6b..cab60e5c2 100644
--- a/app/core/initialize.coffee
+++ b/app/core/initialize.coffee
@@ -13,6 +13,7 @@ channelSchemas =
   'tome': require 'schemas/subscriptions/tome'
   'god': require 'schemas/subscriptions/god'
   'scripts': require 'schemas/subscriptions/scripts'
+  'web-dev': require 'schemas/subscriptions/web-dev'
   'world': require 'schemas/subscriptions/world'
 
 definitionSchemas =
diff --git a/app/schemas/subscriptions/tome.coffee b/app/schemas/subscriptions/tome.coffee
index cff846e0c..d9d5f10d3 100644
--- a/app/schemas/subscriptions/tome.coffee
+++ b/app/schemas/subscriptions/tome.coffee
@@ -89,7 +89,7 @@ module.exports =
   'tome:problems-updated': c.object {title: 'Problems Updated', description: 'Published when problems have been updated', required: ['spell', 'problems', 'isCast']},
     spell: {type: 'object'}
     problems: {type: 'array'}
-    isCast: {type: 'boolean'}
+    isCast: {type: 'boolean', description: 'Whether the code has been Run yet. Sometimes determines if error displays as just annotation or as full banner.'}
 
   'tome:change-language': c.object {title: 'Tome Change Language', description: 'Published when the Tome should update its programming language', required: ['language']},
     language: {type: 'string'}
diff --git a/app/schemas/subscriptions/web-dev.coffee b/app/schemas/subscriptions/web-dev.coffee
new file mode 100644
index 000000000..4bd8c07b7
--- /dev/null
+++ b/app/schemas/subscriptions/web-dev.coffee
@@ -0,0 +1,9 @@
+c = require 'schemas/schemas'
+
+module.exports =
+  'web-dev:error': c.object {title: 'Web Dev Error', description: 'Published when an uncaught error occurs in the web-dev iFrame', required: []},
+    message: { type: 'string' }
+    url: { type: 'string', description: 'URL of the host iFrame' }
+    line: { type: 'integer', description: 'Line number of the start of the code that threw the exception (relative to its <script> tag!)' }
+    column: { type: 'integer', description: 'Column number of the start of the code that threw the exception' }
+    error: { type: 'string', description: 'The .toString of the originally thrown exception' }
diff --git a/app/styles/play/level/tome/spell.sass b/app/styles/play/level/tome/spell.sass
index b3928a4b3..ba6611951 100644
--- a/app/styles/play/level/tome/spell.sass
+++ b/app/styles/play/level/tome/spell.sass
@@ -188,10 +188,6 @@
     .ace_gutter-cell.ace_error
       background-image: url(data:image/png;base64,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)
 
-    // NOTE! This hides all info annotations because removing specific annotations with listeners means they flicker. This hides that flicker. see: SpellView.onChangeAnnotation
-    .ace_gutter-cell.ace_info
-      background-image: none
-
     .ace_marker-layer
       .ace_bracket
         // Override faint gray
diff --git a/app/views/play/level/WebSurfaceView.coffee b/app/views/play/level/WebSurfaceView.coffee
index 56d2d7559..d737a662d 100644
--- a/app/views/play/level/WebSurfaceView.coffee
+++ b/app/views/play/level/WebSurfaceView.coffee
@@ -88,8 +88,11 @@ module.exports = class WebSurfaceView extends CocoView
     switch event.data.type
       when 'goals-updated'
         Backbone.Mediator.publish 'god:new-html-goal-states', goalStates: event.data.goalStates, overallStatus: event.data.overallStatus
+      when 'error'
+        # NOTE: The line number in this is relative to the script tag, not the user code. The offset is added in SpellView.
+        Backbone.Mediator.publish 'web-dev:error', _.pick(event.data, ['message', 'line', 'column', 'url'])
       else
-        console.warn 'Unknown message type', event.data.type, 'for message', e, 'from origin', origin
+        console.warn 'Unknown message type', event.data.type, 'for message', event, 'from origin', origin
 
   destroy: ->
     window.removeEventListener 'message', @onIframeMessage
diff --git a/app/views/play/level/tome/Problem.coffee b/app/views/play/level/tome/Problem.coffee
index 9b5b22fa8..1ebbfeab2 100644
--- a/app/views/play/level/tome/Problem.coffee
+++ b/app/views/play/level/tome/Problem.coffee
@@ -1,43 +1,96 @@
 Range = ace.require('ace/range').Range
 
+# This class can either wrap an AetherProblem,
+# or act as a general runtime error container for web-dev iFrame errors.
+# TODO: Use subclasses? Might need a factory pattern for that (bleh)
 module.exports = class Problem
   annotation: null
   markerRange: null
-  constructor: (@aether, @aetherProblem, @ace, isCast=false, @levelID) ->
-    @buildAnnotation()
-    @buildMarkerRange() if isCast
+  # Construction with AetherProblem will include all but `error`
+  # Construction with a standard error will have `error`, `isCast`, `levelID`, `ace`
+  constructor: ({ @aether, @aetherProblem, @ace, isCast=false, @levelID, error, userCodeHasChangedSinceLastCast }) ->
+    if @aetherProblem
+      @annotation = @buildAnnotationFromAetherProblem(@aetherProblem)
+      { @lineMarkerRange, @textMarkerRange } = @buildMarkerRangesFromAetherProblem(@aetherProblem) if isCast
+
+      { @level, @range, @message, @hint, @userInfo } = @aetherProblem
+      { @row, @column: col } = @aetherProblem.range?[0]
+      @createdBy = 'aether'
+    else
+      unless userCodeHasChangedSinceLastCast
+        @annotation = @buildAnnotationFromWebDevError(error)
+        { @lineMarkerRange, @textMarkerRange } = @buildMarkerRangesFromWebDevError(error)
+
+      @level = 'error'
+      @row = error.line
+      @column = error.column
+      @message = error.message or 'Unknown Error'
+      if error.line and not userCodeHasChangedSinceLastCast
+        @message = "Line #{error.line + 1}: " + @message # Ace's gutter numbers are 1-indexed but annotation.rows are 0-indexed
+      if userCodeHasChangedSinceLastCast
+        @hint = "This error was generated by old code — Try running your new code first."
+      else
+        @hint = undefined
+      @userInfo = undefined
+      @createdBy = 'web-dev-iframe'
+      # TODO: Include runtime/transpile error types depending on something?
+
     # TODO: get ACE screen line, too, for positioning, since any multiline "lines" will mess up positioning
     Backbone.Mediator.publish("problem:problem-created", line: @annotation.row, text: @annotation.text) if application.isIPadApp
 
+  isEqual: (problem) ->
+    _.all ['row', 'column', 'level', 'column', 'message', 'hint'], (attr) =>
+      @[attr] is problem[attr]
+
   destroy: ->
     @removeMarkerRanges()
     @userCodeProblem.off() if @userCodeProblem
 
-  buildAnnotation: ->
-    return unless @aetherProblem.range
-    text = @aetherProblem.message.replace /^Line \d+: /, ''
-    start = @aetherProblem.range[0]
-    @annotation =
+  buildAnnotationFromWebDevError: (error) ->
+    {
+      row: error.line
+      column: error.column
+      raw: error.message
+      text: error.message
+      type: 'error'
+      createdBy: 'web-dev-iframe'
+    }
+
+  buildAnnotationFromAetherProblem: (aetherProblem) ->
+    return unless aetherProblem.range
+    text = aetherProblem.message.replace /^Line \d+: /, ''
+    start = aetherProblem.range[0]
+    {
       row: start.row,
       column: start.col,
       raw: text,
       text: text,
       type: @aetherProblem.level ? 'error'
       createdBy: 'aether'
+    }
 
-  buildMarkerRange: ->
-    return unless @aetherProblem.range
-    [start, end] = @aetherProblem.range
-    textClazz = "problem-marker-#{@aetherProblem.level}"
-    @textMarkerRange = new Range start.row, start.col, end.row, end.col
-    @textMarkerRange.start = @ace.getSession().getDocument().createAnchor @textMarkerRange.start
-    @textMarkerRange.end = @ace.getSession().getDocument().createAnchor @textMarkerRange.end
-    @textMarkerRange.id = @ace.getSession().addMarker @textMarkerRange, textClazz, 'text'
+  buildMarkerRangesFromWebDevError: (error) ->
+    lineMarkerRange = new Range error.line, 0, error.line, 1
+    lineMarkerRange.start = @ace.getSession().getDocument().createAnchor lineMarkerRange.start
+    lineMarkerRange.end = @ace.getSession().getDocument().createAnchor lineMarkerRange.end
+    lineMarkerRange.id = @ace.getSession().addMarker lineMarkerRange, 'problem-line', 'fullLine'
+    textMarkerRange = undefined # We don't get any per-character info from standard errors
+    { lineMarkerRange, textMarkerRange }
+
+  buildMarkerRangesFromAetherProblem: (aetherProblem) ->
+    return {} unless aetherProblem.range
+    [start, end] = aetherProblem.range
+    textClazz = "problem-marker-#{aetherProblem.level}"
+    textMarkerRange = new Range start.row, start.col, end.row, end.col
+    textMarkerRange.start = @ace.getSession().getDocument().createAnchor textMarkerRange.start
+    textMarkerRange.end = @ace.getSession().getDocument().createAnchor textMarkerRange.end
+    textMarkerRange.id = @ace.getSession().addMarker textMarkerRange, textClazz, 'text'
     lineClazz = "problem-line"
-    @lineMarkerRange = new Range start.row, start.col, end.row, end.col
-    @lineMarkerRange.start = @ace.getSession().getDocument().createAnchor @lineMarkerRange.start
-    @lineMarkerRange.end = @ace.getSession().getDocument().createAnchor @lineMarkerRange.end
-    @lineMarkerRange.id = @ace.getSession().addMarker @lineMarkerRange, lineClazz, 'fullLine'
+    lineMarkerRange = new Range start.row, start.col, end.row, end.col
+    lineMarkerRange.start = @ace.getSession().getDocument().createAnchor lineMarkerRange.start
+    lineMarkerRange.end = @ace.getSession().getDocument().createAnchor lineMarkerRange.end
+    lineMarkerRange.id = @ace.getSession().addMarker lineMarkerRange, lineClazz, 'fullLine'
+    { lineMarkerRange, textMarkerRange }
 
   removeMarkerRanges: ->
     if @textMarkerRange
diff --git a/app/views/play/level/tome/ProblemAlertView.coffee b/app/views/play/level/tome/ProblemAlertView.coffee
index c9ffc1363..f63e1e13c 100644
--- a/app/views/play/level/tome/ProblemAlertView.coffee
+++ b/app/views/play/level/tome/ProblemAlertView.coffee
@@ -20,10 +20,10 @@ module.exports = class ProblemAlertView extends CocoView
     'click': -> Backbone.Mediator.publish 'tome:focus-editor', {}
 
   constructor: (options) ->
+    @supermodel = options.supermodel # Has to go before super so events are hooked up
     super options
     @level = options.level
     @session = options.session
-    @supermodel = options.supermodel
     if options.problem?
       @problem = options.problem
       @onWindowResize()
@@ -38,31 +38,31 @@ module.exports = class ProblemAlertView extends CocoView
   afterRender: ->
     super()
     if @problem?
-      @$el.addClass('alert').addClass("alert-#{@problem.aetherProblem.level}").hide().fadeIn('slow')
-      @$el.addClass('no-hint') unless @problem.aetherProblem.hint
+      @$el.addClass('alert').addClass("alert-#{@problem.level}").hide().fadeIn('slow')
+      @$el.addClass('no-hint') unless @problem.hint
       @playSound 'error_appear'
 
   setProblemMessage: ->
     if @problem?
       format = (s) -> marked(s.replace(/</g, '&lt;').replace(/>/g, '&gt;')) if s?
-      message = @problem.aetherProblem.message
+      message = @problem.message
       # Add time to problem message if hint is for a missing null check
       # NOTE: This may need to be updated with Aether error hint changes
-      if @problem.aetherProblem.hint? and /(?:null|undefined)/.test @problem.aetherProblem.hint
-        age = @problem.aetherProblem.userInfo?.age
+      if @problem.hint? and /(?:null|undefined)/.test @problem.hint
+        age = @problem.userInfo?.age
         if age?
           if /^Line \d+:/.test message
             message = message.replace /^(Line \d+)/, "$1, time #{age.toFixed(1)}"
           else
             message = "Time #{age.toFixed(1)}: #{message}"
       @message = format message
-      @hint = format @problem.aetherProblem.hint
+      @hint = format @problem.hint
 
   onShowProblemAlert: (data) ->
     return unless $('#code-area').is(":visible")
     if @problem?
-      if @$el.hasClass "alert-#{@problem.aetherProblem.level}"
-        @$el.removeClass "alert-#{@problem.aetherProblem.level}"
+      if @$el.hasClass "alert-#{@problem.level}"
+        @$el.removeClass "alert-#{@problem.level}"
       if @$el.hasClass "no-hint"
         @$el.removeClass "no-hint"
     @problem = data.problem
diff --git a/app/views/play/level/tome/Spell.coffee b/app/views/play/level/tome/Spell.coffee
index 5b8f14532..1db486b82 100644
--- a/app/views/play/level/tome/Spell.coffee
+++ b/app/views/play/level/tome/Spell.coffee
@@ -145,6 +145,7 @@ module.exports = class Spell
       @thang?.aether.transpile source
     null
 
+  # NOTE: By default, I think this compares the current source code with the source *last saved to the server* (not the last time it was run)
   hasChanged: (newSource=null, currentSource=null) ->
     (newSource ? @originalSource) isnt (currentSource ? @source)
 
diff --git a/app/views/play/level/tome/SpellView.coffee b/app/views/play/level/tome/SpellView.coffee
index fe8d07c0a..2dc095b82 100644
--- a/app/views/play/level/tome/SpellView.coffee
+++ b/app/views/play/level/tome/SpellView.coffee
@@ -49,10 +49,12 @@ module.exports = class SpellView extends CocoView
     'tome:insert-snippet': 'onInsertSnippet'
     'tome:spell-beautify': 'onSpellBeautify'
     'tome:maximize-toggled': 'onMaximizeToggled'
+    'tome:problems-updated': 'onProblemsUpdated'
     'script:state-changed': 'onScriptStateChange'
     'playback:ended-changed': 'onPlaybackEndedChanged'
     'level:contact-button-pressed': 'onContactButtonPressed'
     'level:show-victory': 'onShowVictory'
+    'web-dev:error': 'onWebDevError'
 
   events:
     'mouseout': 'onMouseOut'
@@ -87,6 +89,14 @@ module.exports = class SpellView extends CocoView
     @destroyAceEditor(@ace)
     @ace = ace.edit @$el.find('.ace')[0]
     @aceSession = @ace.getSession()
+    # Override setAnnotations so the Ace html worker doesn't clobber our annotations
+    @reallySetAnnotations = @aceSession.setAnnotations.bind(@aceSession)
+    @aceSession.setAnnotations = (annotations) =>
+      previousAnnotations = @aceSession.getAnnotations()
+      newAnnotations = _.filter previousAnnotations, (annotation) -> annotation.createdBy? # Keep the ones we generated
+        .concat _.reject annotations, (annotation) -> # Ignore this particular info-annotation the html worker generates
+          annotation.text is 'Start tag seen without seeing a doctype first. Expected e.g. <!DOCTYPE html>.'
+      @reallySetAnnotations newAnnotations
     @aceDoc = @aceSession.getDocument()
     @aceSession.setUseWorker @spell.language in @languagesThatUseWorkers
     @aceSession.setMode utils.aceEditModes[@spell.language]
@@ -94,7 +104,6 @@ module.exports = class SpellView extends CocoView
     @aceSession.setUseWrapMode true
     @aceSession.setNewLineMode 'unix'
     @aceSession.setUseSoftTabs true
-    @aceSession.on 'changeAnnotation', @onChangeAnnotation
     @ace.setTheme 'ace/theme/textmate'
     @ace.setDisplayIndentGuides false
     @ace.setShowPrintMargin false
@@ -670,7 +679,10 @@ module.exports = class SpellView extends CocoView
     cast = @$el.parent().length
     @recompile cast, e.realTime
     @focus() if cast
-    @updateHTML create: true if @options.level.isType('web-dev')
+    if @options.level.isType('web-dev')
+      @sourceAtLastCast = @getSource()
+      @ace.setStyle 'spell-cast'
+      @updateHTML create: true
 
   onCodeReload: (e) ->
     return unless e.spell is @spell or not e.spell
@@ -736,6 +748,11 @@ module.exports = class SpellView extends CocoView
   onCursorActivity: =>  # Used to refresh autocast delay; doesn't do anything at the moment.
 
   updateHTML: (options={}) =>
+    # TODO: Merge with onSpellChanged
+    # NOTE: Consider what goes in onManualCast only
+    if @spell.hasChanged(@spell.getSource(), @sourceAtLastCast)
+      @ace.unsetStyle 'spell-cast' # NOTE: Doesn't do anything for web-dev as of this writing, including for consistency
+    @clearWebDevErrors()
     Backbone.Mediator.publish 'tome:html-updated', html: @spell.constructHTML(@getSource()), create: Boolean(options.create)
 
   # Design for a simpler system?
@@ -770,6 +787,7 @@ module.exports = class SpellView extends CocoView
       # Now that that's figured out, perform the update.
       # The web worker Aether won't track state, so don't have to worry about updating it
       finishUpdatingAether = (aether) =>
+        @clearAetherDisplay() # In case problems were added since last clearing
         @displayAether aether, codeIsAsCast
         @lastUpdatedAetherSpellThang = @spellThang
         @guessWhetherFinished aether if fromCodeChange
@@ -796,57 +814,92 @@ module.exports = class SpellView extends CocoView
       else
         finishUpdatingAether(aether)
 
-  # NOTE! Because this alone causes the doctype annotation to flicker,
-  # all info annotations have been hidden with CSS in spell.sass
-  # If we ever want info annotations back, we need to remove that.
-  #
-  # This function itself removes the unwanted annotations on a later tick.
-  onChangeAnnotation: (event, session) ->
-    unfilteredAnnotations = session.getAnnotations()
-    filteredAnnotations = _.reject unfilteredAnnotations, (annotation) ->
-      annotation.text is 'Start tag seen without seeing a doctype first. Expected e.g. <!DOCTYPE html>.'
-    if filteredAnnotations.length < unfilteredAnnotations.length
-      session.setAnnotations(filteredAnnotations)
-
-  # Clear annotations and highlights generated by Aether, but not by the ACE worker
+  # Each problem-generating piece (aether, web-dev, ace html worker) clears its own problems/annotations
   clearAetherDisplay: ->
-    problem.destroy() for problem in @problems
-    @problems = []
-    nonAetherAnnotations = _.reject @aceSession.getAnnotations(), (annotation) -> annotation.createdBy is 'aether'
-    @aceSession.setAnnotations nonAetherAnnotations
+    @clearProblemsCreatedBy 'aether'
     @highlightCurrentLine {}  # This'll remove all highlights
 
+  clearWebDevErrors: ->
+    @clearProblemsCreatedBy 'web-dev-iframe'
+
+  clearProblemsCreatedBy: (createdBy) ->
+    nonAetherAnnotations = _.reject @aceSession.getAnnotations(), (annotation) -> annotation.createdBy is createdBy
+    @reallySetAnnotations nonAetherAnnotations
+
+    problemsToClear = _.filter @problems, (p) -> p.createdBy is createdBy
+    problemsToClear.forEach (problem) -> problem.destroy()
+    @problems = _.difference @problems, problemsToClear
+    Backbone.Mediator.publish 'tome:problems-updated', spell: @spell, problems: @problems, isCast: false
+
+  convertAetherProblems: (aether, aetherProblems, isCast) ->
+    # TODO: Functional-ify
+    _.unique(aetherProblems, (p) -> p.userInfo?.key).map (aetherProblem) =>
+      new Problem { aether, aetherProblem, @ace, isCast, levelID: @options.levelID }
+
   displayAether: (aether, isCast=false) ->
     @displayedAether = aether
     isCast = isCast or not _.isEmpty(aether.metrics) or _.some aether.getAllProblems(), {type: 'runtime'}
-    problem.destroy() for problem in @problems  # Just in case another problem was added since clearAetherDisplay() ran.
-    @problems = []
     annotations = @aceSession.getAnnotations()
-    seenProblemKeys = {}
-    for aetherProblem, problemIndex in aether.getAllProblems()
-      continue if key = aetherProblem.userInfo?.key and key of seenProblemKeys
-      seenProblemKeys[key] = true if key
-      @problems.push problem = new Problem aether, aetherProblem, @ace, isCast, @options.levelID
-      if isCast and problemIndex is 0
-        if problem.aetherProblem.range?
-          lineOffsetPx = 0
-          for i in [0...problem.aetherProblem.range[0].row]
-            lineOffsetPx += @aceSession.getRowLength(i) * @ace.renderer.lineHeight
-          lineOffsetPx -= @ace.session.getScrollTop()
-        Backbone.Mediator.publish 'tome:show-problem-alert', problem: problem, lineOffsetPx: Math.max lineOffsetPx, 0
-      @saveUserCodeProblem(aether, aetherProblem) if isCast
-      annotations.push problem.annotation if problem.annotation
+
+    newProblems = @convertAetherProblems(aether, aether.getAllProblems(), isCast)
+    annotations.push problem.annotation for problem in newProblems when problem.annotation
+    if isCast
+      @displayProblemBanner(newProblems[0]) if newProblems[0]
+      @saveUserCodeProblem(aether, problem.aetherProblem) for problem in newProblems
+    @problems = @problems.concat(newProblems)
+
     @aceSession.setAnnotations annotations
     @highlightCurrentLine aether.flow unless _.isEmpty aether.flow
     #console.log '  and we could do the metrics', aether.metrics unless _.isEmpty aether.metrics
     #console.log '  and we could do the style', aether.style unless _.isEmpty aether.style
     #console.log '  and we could do the visualization', aether.visualization unless _.isEmpty aether.visualization
-    # Could use the user-code-problem style... or we could leave that to other places.
-    @ace[if @problems.length then 'setStyle' else 'unsetStyle'] 'user-code-problem'
-    @ace[if isCast then 'setStyle' else 'unsetStyle'] 'spell-cast'
     Backbone.Mediator.publish 'tome:problems-updated', spell: @spell, problems: @problems, isCast: isCast
     @ace.resize()
 
+  # Tell ProblemAlertView to display this problem (only)
+  displayProblemBanner: (problem) ->
+    lineOffsetPx = 0
+    if problem.row?
+      for i in [0...problem.row]
+        lineOffsetPx += @aceSession.getRowLength(i) * @ace.renderer.lineHeight
+      lineOffsetPx -= @ace.session.getScrollTop()
+    Backbone.Mediator.publish 'tome:show-problem-alert', problem: problem, lineOffsetPx: Math.max lineOffsetPx, 0
+
+  # Gets the number of lines before the start of <script> content in the usercode
+  # Because Errors report their line number relative to the <script> tag
+  linesBeforeScript: (html) ->
+    # TODO: refactor, make it work with multiple scripts. What to do when error is in level-creator's code?
+    _.size(html.split('<script>')[0].match(/\n/g))
+
+  addAnnotation: (annotation) ->
+    annotations = @aceSession.getAnnotations()
+    annotations.push annotation
+    @reallySetAnnotations annotations
+
+  # Handle errors from the web-dev iframe asynchronously
+  onWebDevError: (error) ->
+    # TODO: Refactor this and the Aether problem flow to share as much as possible.
+    # TODO: Handle when the error is in our code, not theirs
+    # Compensate for line number being relative to <script> tag
+    offsetError = _.merge {}, error, { line: error.line + @linesBeforeScript(@getSource()) }
+    userCodeHasChangedSinceLastCast = @spell.hasChanged(@spell.getSource(), @sourceAtLastCast)
+    problem = new Problem({ error: offsetError, @ace, levelID: @options.levelID, userCodeHasChangedSinceLastCast })
+    # Ignore the Problem if we already know about it
+    if _.any(@problems, (preexistingProblem) -> problem.isEqual(preexistingProblem))
+      problem.destroy()
+    else # Ok, the problem is worth keeping
+      @problems.push problem
+      @displayProblemBanner(problem)
+
+      # @saveUserCodeProblem(aether, aetherProblem) # TODO: Enable saving of web-dev user code problems
+      @addAnnotation(problem.annotation) if problem.annotation
+      Backbone.Mediator.publish 'tome:problems-updated', spell: @spell, problems: @problems, isCast: false
+
+  onProblemsUpdated: ({ spell, problems, isCast }) ->
+    # This just handles some ace styles for now; other things handle @problems changes elsewhere
+    @ace[if problems.length then 'setStyle' else 'unsetStyle'] 'user-code-problem'
+    @ace[if isCast then 'setStyle' else 'unsetStyle'] 'spell-cast' # Does this still do anything?
+
   saveUserCodeProblem: (aether, aetherProblem) ->
     # Skip duplicate problems
     hashValue = aether.raw + aetherProblem.message
@@ -939,6 +992,7 @@ module.exports = class SpellView extends CocoView
       @spell.thang.aether[key] = value
 
   onSpellChanged: (e) ->
+    # TODO: Merge with updateHTML
     @spellHasChanged = true
 
   onSessionWillSave: (e) ->
diff --git a/server/commons/database.coffee b/server/commons/database.coffee
index 218ec51f9..ceea28b6a 100644
--- a/server/commons/database.coffee
+++ b/server/commons/database.coffee
@@ -206,4 +206,3 @@ module.exports =
       return false if index is -1
       return false if delta.deltaPath[index+1] in ['en', 'en-US', 'en-GB']  # English speakers are most likely just spamming, so always treat those as patches, not saves.
       return true
-
diff --git a/test/app/views/play/level/tome/Problem.spec.coffee b/test/app/views/play/level/tome/Problem.spec.coffee
index 09b96bdbe..84affeb9a 100644
--- a/test/app/views/play/level/tome/Problem.spec.coffee
+++ b/test/app/views/play/level/tome/Problem.spec.coffee
@@ -29,7 +29,7 @@ describe 'Problem', ->
 
   # TODO: Problems are no longer saved when creating Problems; instead it's in SpellView. Update tests?
   xit 'save user code problem', ->
-    new Problem aether, aetherProblem, ace, false, true, levelID
+    new Problem {aether, aetherProblem, ace, isCast: false, levelID}
     expect(jasmine.Ajax.requests.count()).toBe(1)
 
     request = jasmine.Ajax.requests.mostRecent()
@@ -49,7 +49,7 @@ describe 'Problem', ->
 
   xit 'save user code problem no range', ->
     aetherProblem.range = null
-    new Problem aether, aetherProblem, ace, false, true, levelID
+    new Problem {aether, aetherProblem, ace, isCast: false, levelID}
     expect(jasmine.Ajax.requests.count()).toBe(1)
 
     request = jasmine.Ajax.requests.mostRecent()
@@ -73,7 +73,7 @@ describe 'Problem', ->
     aether.raw = "this.say('hi');\nthis.sad\n('bye');"
     aetherProblem.range = [ { row: 1 }, { row: 2 } ]
 
-    new Problem aether, aetherProblem, ace, false, true, levelID
+    new Problem {aether, aetherProblem, ace, isCast: false, levelID}
     expect(jasmine.Ajax.requests.count()).toBe(1)
 
     request = jasmine.Ajax.requests.mostRecent()