diff --git a/chrome/content/zotero/longTagFixer.js b/chrome/content/zotero/longTagFixer.js
new file mode 100644
index 000000000..8113de980
--- /dev/null
+++ b/chrome/content/zotero/longTagFixer.js
@@ -0,0 +1,185 @@
+var Zotero_Long_Tag_Fixer = new function () {
+	var _oldTag = window.arguments[0];
+	var _callback = window.arguments[1];
+	
+	this.init = function () {
+		document.getElementById('zotero-old-tag').value = _oldTag;
+		
+		var lastMode = Zotero.Prefs.get('lastLongTagMode');
+		if (!lastMode) {
+			lastMode = 0;
+		}
+		this.switchMode(lastMode);
+	}
+	
+	this.switchMode = function (index) {
+		var dialog = document.getElementById('zotero-long-tag-fixer');
+		
+		document.getElementById('zotero-new-tag-actions').selectedIndex = index;
+		
+		// TODO: localize
+		switch (index) {
+			case 0:
+				var buttonLabel = "Save Tags";
+				this.updateTagList();
+				break;
+				
+			case 1:
+				var buttonLabel = "Save Tag";
+				document.getElementById('zotero-new-tag-editor').value = _oldTag;
+				this.updateEditLength(_oldTag.length)
+				break;
+				
+			case 2:
+				var buttonLabel = "Delete Tag";
+				dialog.getButton('accept').disabled = false;
+				break;
+		}
+		
+		document.getElementById('zotero-long-tag-fixer').getButton('accept').label = buttonLabel;
+		
+		Zotero.Prefs.set('lastLongTagMode', index);
+	}
+	
+	
+	/**
+	 * Split tags and populate list
+	 */
+	this.updateTagList = function () {
+		var listbox = document.getElementById('zotero-new-tag-list');
+		while (listbox.childNodes.length) {
+			listbox.removeChild(listbox.lastChild);
+		}
+		
+		var delimiter = document.getElementById('zotero-old-tag-delimiter').value;
+		if (delimiter) {
+			var re = new RegExp("\\s*" + delimiter + "\\s*");
+			var tags = _oldTag.split(re);
+		}
+		
+		var acceptButton = document.getElementById('zotero-long-tag-fixer').getButton('accept');
+		if (!delimiter || tags.length < 2) {
+			acceptButton.disabled = true;
+			return;
+		}
+		else {
+			acceptButton.disabled = false;
+		}
+		
+		tags.sort();
+		for (var i=0; i<tags.length; i++) {
+			if (i==0 || tags[i] == tags[i-1]) {
+				continue;
+			}
+			var li = listbox.appendItem(tags[i]);
+			li.setAttribute('type', 'checkbox');
+			li.setAttribute('checked', 'true');
+		}
+	}
+	
+	
+	this.deselectAll = function () {
+		var lis = document.getElementById('zotero-new-tag-list').getElementsByTagName('listitem');
+		for (var i=0; i<lis.length; i++) {
+			lis[i].checked = false;
+		}
+	}
+	
+	
+	this.selectAll = function () {
+		var lis = document.getElementById('zotero-new-tag-list').getElementsByTagName('listitem');
+		for (var i=0; i<lis.length; i++) {
+			lis[i].checked = true;
+		}
+	}
+	
+	
+	this.updateEditLength = function (len) {
+		document.getElementById('zotero-new-tag-character-count').value = len;
+		var invalid = len == 0 || len > 255;
+		document.getElementById('zotero-new-tag-characters').setAttribute('invalid', invalid);
+		document.getElementById('zotero-long-tag-fixer').getButton('accept').disabled = invalid;
+	}
+	
+	
+	this.cancel = function () {
+		if (_callback) {
+			_callback(false);
+		}
+	}
+	
+	
+	this.save = function () {
+		try {
+		
+		var index = document.getElementById('zotero-new-tag-actions').selectedIndex;
+		
+		// Search for all matching tags across all libraries
+		var sql = "SELECT tagID FROM tags WHERE name=?";
+		var oldTagIDs = Zotero.DB.columnQuery(sql, _oldTag);
+		
+		switch (index) {
+			// Split
+			case 0:
+				// Get checked tags
+				var listbox = document.getElementById('zotero-new-tag-list');
+				var len = Zotero.isFx3 ? listbox.childNodes.length : listbox.childElementCount;
+				var newTags = [];
+				for (var i=0; i<len; i++) {
+					var li = listbox.childNodes[i];
+					if (li.getAttribute('checked') == 'true') {
+						newTags.push(li.getAttribute('label'));
+					}
+				}
+				
+				Zotero.DB.beginTransaction();
+				
+				// Add new tags to all items linked to each matching old tag
+				for (var i=0; i<oldTagIDs.length; i++) {
+					var tag = Zotero.Tags.get(oldTagIDs[i]);
+					var items = tag.getLinkedItems();
+					if (items) {
+						for (var j=0; j<items.length; j++) {
+							items[j].addTags(newTags, tag.type);
+						}
+					}
+				}
+				
+				// Remove old tags
+				Zotero.Tags.erase(oldTagIDs);
+				Zotero.Tags.purge();
+				Zotero.DB.commitTransaction();
+				break;
+			
+			// Edit
+			case 1:
+				var value = document.getElementById('zotero-new-tag-editor').value;
+				Zotero.DB.beginTransaction();
+				for (var i=0; i<oldTagIDs.length; i++) {
+					var tag = Zotero.Tags.get(oldTagIDs[i]);
+					tag.name = value;
+					tag.save();
+				}
+				Zotero.DB.commitTransaction();
+				break;
+			
+			// Delete
+			case 2:
+				Zotero.DB.beginTransaction();
+				Zotero.Tags.erase(oldTagIDs);
+				Zotero.Tags.purge();
+				Zotero.DB.commitTransaction();
+				break;
+		}
+		
+		if (_callback) {
+			_callback(true);
+		}
+		
+		}
+		catch (e) {
+			Zotero.debug(e);
+			throw (e);
+		}
+	}
+}
diff --git a/chrome/content/zotero/longTagFixer.xul b/chrome/content/zotero/longTagFixer.xul
new file mode 100644
index 000000000..8d76623ec
--- /dev/null
+++ b/chrome/content/zotero/longTagFixer.xul
@@ -0,0 +1,74 @@
+<?xml version="1.0"?>
+<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>
+<?xml-stylesheet href="chrome://zotero/skin/longTagFixer.css" type="text/css"?>
+
+<dialog id="zotero-long-tag-fixer" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+		buttons="accept,cancel"
+		onload="Zotero_Long_Tag_Fixer.init()"
+		ondialogcancel="Zotero_Long_Tag_Fixer.cancel()"
+		ondialogaccept="Zotero_Long_Tag_Fixer.save()"
+		title="Sync Error"> <!-- TODO: localize -->
+	
+	<script src="include.js"/>
+	<script src="longTagFixer.js"/>
+	
+	<!-- TODO: localize -->
+	<label value="The following tag in your Zotero library is too long to sync to the server:"/>
+	<groupbox>
+		<textbox id="zotero-old-tag" multiline="true" rows="4" readonly="true" class="plain"/>
+	</groupbox>
+	<label>Synced tags must be shorter than 256 characters.</label>
+	
+	<separator class="thin"/>
+	
+	<label value="You can either split the tag into multiple tags, edit the tag manually to shorten it, or delete it."/>
+	
+	<separator/>
+	
+	<tabbox id="zotero-new-tag-actions">
+		<tabs oncommand="Zotero_Long_Tag_Fixer.switchMode(this.selectedIndex)">
+			<tab label="Split"/>
+			<tab label="Edit"/>
+			<tab label="Delete"/>
+		</tabs>
+		<tabpanels>
+			<!-- Split -->
+			<vbox>
+				<hbox align="center">
+					<label>Split at the </label>
+					<textbox id="zotero-old-tag-delimiter" size="1" maxLength="1" value=";"
+						oninput="Zotero_Long_Tag_Fixer.updateTagList()"/>
+					<label>character</label>
+				</hbox>
+				
+				<separator class="thin"/>
+				
+				<listbox id="zotero-new-tag-list" rows="8"/>
+				
+				<separator class="thin"/>
+				
+				<label value="Unchecked tags will not be saved."/>
+				
+				<hbox>
+					<button label="Deselect All" oncommand="Zotero_Long_Tag_Fixer.deselectAll()"/>
+					<button label="Select All" oncommand="Zotero_Long_Tag_Fixer.selectAll()"/>
+				</hbox>
+			</vbox>
+			
+			<!-- Edit -->
+			<vbox>
+				<textbox id="zotero-new-tag-editor" multiline="true" flex="1"
+					oninput="Zotero_Long_Tag_Fixer.updateEditLength(this.value.length)"/>
+				<hbox id="zotero-new-tag-characters">
+					<label id="zotero-new-tag-character-count"/>
+					<label value="characters"/>
+				</hbox>
+			</vbox>
+			
+			<!-- Delete -->
+			<vbox align="center" pack="center">
+				<label value="The tag will be deleted from all items."/>
+			</vbox>
+		</tabpanels>
+	</tabbox>
+</dialog>
diff --git a/chrome/content/zotero/xpcom/sync.js b/chrome/content/zotero/xpcom/sync.js
index 97d998024..063608e29 100644
--- a/chrome/content/zotero/xpcom/sync.js
+++ b/chrome/content/zotero/xpcom/sync.js
@@ -951,6 +951,8 @@ Zotero.Sync.Server = new function () {
 				_error(response.firstChild.firstChild.nodeValue);
 			}
 			
+			_sessionLock = true;
+			
 			// Strip XML declaration and convert to E4X
 			var xml = new XML(xmlhttp.responseText.replace(/<\?xml.*\?>/, ''));
 			
@@ -1463,15 +1465,40 @@ Zotero.Sync.Server = new function () {
 		}
 		
 		
-		if (firstChild.localName == 'error' && firstChild.getAttribute('code') == 'ITEM_MISSING') {
-			var [libraryID, key] = firstChild.getAttribute('missingItem').split('/');
-			if (libraryID == Zotero.libraryID) {
-				libraryID = null;
-			}
-			var item = Zotero.Items.getByLibraryAndKey(libraryID, key);
-			if (item) {
-				Zotero.DB.rollbackAllTransactions();
-				item.updateClientDateModified();
+		if (firstChild.localName == 'error') {
+			switch (firstChild.getAttribute('code')) {
+				case 'ITEM_MISSING':
+					var [libraryID, key] = firstChild.getAttribute('missingItem').split('/');
+					if (libraryID == Zotero.libraryID) {
+						libraryID = null;
+					}
+					var item = Zotero.Items.getByLibraryAndKey(libraryID, key);
+					if (item) {
+						Zotero.DB.rollbackAllTransactions();
+						item.updateClientDateModified();
+					}
+					break;
+				
+				case 'TAG_TOO_LONG':
+					var tag = xmlhttp.responseXML.firstChild.getElementsByTagName('tag');
+					if (tag.length) {
+						Zotero.DB.rollbackAllTransactions();
+						
+						var tag = tag[0].firstChild.nodeValue;
+						setTimeout(function () {
+							var callback = function (success) {
+								if (success) {
+									Zotero.Sync.Runner.sync();
+								}
+							};
+							
+							var wm = Components.classes["@mozilla.org/appshell/window-mediator;1"]
+									   .getService(Components.interfaces.nsIWindowMediator);
+							var lastWin = wm.getMostRecentWindow("navigator:browser");
+							lastWin.openDialog('chrome://zotero/content/longTagFixer.xul', '', 'chrome,modal,centerscreen', tag, callback);
+						}, 1);
+					}
+					break;
 			}
 		}
 	}
@@ -1925,8 +1952,6 @@ Zotero.Sync.Server.Data = new function() {
 									
 								case 'item':
 									var diff = obj.diff(remoteObj, false, ["dateModified"]);
-									Zotero.debug('diff');
-									Zotero.debug(diff);
 									if (!diff) {
 										// Check if creators changed
 										var creatorsChanged = false;
@@ -1944,11 +1969,9 @@ Zotero.Sync.Server.Data = new function() {
 												var r = remoteCreatorStore[Zotero.Creators.getLibraryKeyHash(creator.ref)];
 												// Doesn't include dateModified
 												if (r && !r.equals(creator.ref)) {
-													Zotero.debug('=');
 													creatorsChanged = true;
 													break;
 												}
-												Zotero.debug('-');
 											}
 										}
 										if (!creatorsChanged) {
diff --git a/chrome/skin/default/zotero/longTagFixer.css b/chrome/skin/default/zotero/longTagFixer.css
new file mode 100644
index 000000000..b8dde1891
--- /dev/null
+++ b/chrome/skin/default/zotero/longTagFixer.css
@@ -0,0 +1,8 @@
+#zotero-new-tag-characters[invalid="true"] {
+	color: red;
+}
+
+#zotero-new-tag-character-count {
+	font-weight: bold;
+	margin-right: 0;
+}
diff --git a/defaults/preferences/zotero.js b/defaults/preferences/zotero.js
index 9a7174a2a..2fd17f46f 100644
--- a/defaults/preferences/zotero.js
+++ b/defaults/preferences/zotero.js
@@ -42,6 +42,7 @@ pref("extensions.zotero.lastCreatorFieldMode",0);
 pref("extensions.zotero.lastAbstractExpand",0);
 pref("extensions.zotero.lastRenameAssociatedFile", false);
 pref("extensions.zotero.lastViewedFolder", 'L');
+pref("extensions.zotero.lastLongTagMode", 0);
 
 //Tag Cloud
 pref("extensions.zotero.tagCloud", false);