diff --git a/chrome/content/zotero/xpcom/sync/syncRunner.js b/chrome/content/zotero/xpcom/sync/syncRunner.js
index ce65901b5b..6d30dadca4 100644
--- a/chrome/content/zotero/xpcom/sync/syncRunner.js
+++ b/chrome/content/zotero/xpcom/sync/syncRunner.js
@@ -32,6 +32,7 @@ if (!Zotero.Sync) {
 // Initialized as Zotero.Sync.Runner in zotero.js
 Zotero.Sync.Runner_Module = function (options = {}) {
 	const stopOnError = false;
+	const HTML_NS = 'http://www.w3.org/1999/xhtml';
 	
 	Zotero.defineProperty(this, 'enabled', {
 		get: () => {
@@ -84,7 +85,9 @@ Zotero.Sync.Runner_Module = function (options = {}) {
 	var _lastSyncStatus;
 	var _currentSyncStatusLabel;
 	var _currentLastSyncLabel;
+	var _currentTooltipMessages;
 	var _errors = [];
+	var _tooltipMessages = [];
 	
 	Zotero.addShutdownListener(() => this.stop());
 	
@@ -118,6 +121,7 @@ Zotero.Sync.Runner_Module = function (options = {}) {
 	this._sync = Zotero.Promise.coroutine(function* (options) {
 		// Clear message list
 		_errors = [];
+		_tooltipMessages = [];
 		
 		// Shouldn't be possible because of serial()
 		if (_syncInProgress) {
@@ -1033,7 +1037,8 @@ Zotero.Sync.Runner_Module = function (options = {}) {
 			upgrade: 4,
 			
 			// Skip these
-			animate: -1
+			animate: -1,
+			ignore: -2
 		};
 		var state = false;
 		for (let i = 0; i < errors.length; i++) {
@@ -1265,20 +1270,31 @@ Zotero.Sync.Runner_Module = function (options = {}) {
 		}
 		// Show warning for unknown data that couldn't be saved
 		else if (e.name && e.name == 'ZoteroInvalidDataError') {
-			e.message = Zotero.getString(
-				'sync.error.invalidDataError',
-				[
-					Zotero.Libraries.get(e.libraryID).name,
-					Zotero.clientName
-				]
-			)
-				+ "\n\n"
-				+ Zotero.getString('sync.error.invalidDataError.otherData');
-			e.errorType = 'warning';
-			e.dialogButtonText = Zotero.getString('general.checkForUpdates');
-			e.dialogButtonCallback = () => {
-				Zotero.openCheckForUpdatesWindow();
-			};
+			let library = Zotero.Libraries.get(e.libraryID);
+			let msg = Zotero.getString(
+					'sync.error.invalidDataError',
+					[
+						library.name,
+						Zotero.clientName
+					]
+				)
+					+ "\n\n"
+					+ Zotero.getString('sync.error.invalidDataError.otherData');
+			
+			// Show warning for My Library
+			if (library.libraryType == 'user') {
+				e.message = msg;
+				e.errorType = 'warning';
+				e.dialogButtonText = Zotero.getString('general.checkForUpdates');
+				e.dialogButtonCallback = () => {
+					Zotero.openCheckForUpdatesWindow();
+				};
+			}
+			// Otherwise just show in sync button tooltip
+			else {
+				_addTooltipMessage(msg);
+				e.errorType = 'ignore';
+			}
 		}
 	});
 	
@@ -1299,6 +1315,7 @@ Zotero.Sync.Runner_Module = function (options = {}) {
 			if (!Array.isArray(errors)) {
 				errors = [errors];
 			}
+			errors = errors.filter(o => o.errorType !== 'ignore');
 			var state = this.getPrimaryErrorType(errors);
 		}
 		
@@ -1512,10 +1529,12 @@ Zotero.Sync.Runner_Module = function (options = {}) {
 		if (tooltip) {
 			_currentSyncStatusLabel = tooltip.firstChild.nextSibling;
 			_currentLastSyncLabel = tooltip.firstChild.nextSibling.nextSibling;
+			_currentTooltipMessages = tooltip.querySelector('.sync-button-tooltip-messages');
 		}
 		else {
 			_currentSyncStatusLabel = null;
 			_currentLastSyncLabel = null;
+			_currentTooltipMessages = null;
 		}
 		if (_currentSyncStatusLabel) {
 			_updateSyncStatusLabel();
@@ -1547,7 +1566,12 @@ Zotero.Sync.Runner_Module = function (options = {}) {
 		Zotero.Sync.Data.Local.setAPIKey();
 		yield client.deleteAPIKey();
 	})
-
+	
+	
+	function _addTooltipMessage(msg) {
+		_tooltipMessages.push(msg.replace(/\n+/g, ' '));
+	};
+	
 	
 	function _updateSyncStatusLabel() {
 		if (_lastSyncStatus) {
@@ -1585,6 +1609,19 @@ Zotero.Sync.Runner_Module = function (options = {}) {
 		
 		_currentLastSyncLabel.value = Zotero.getString('sync.status.lastSync') + " " + msg;
 		_currentLastSyncLabel.hidden = false;
+		
+		if (_tooltipMessages.length) {
+			_currentTooltipMessages.textContent = '';
+			for (let message of _tooltipMessages) {
+				let elem = _currentTooltipMessages.ownerDocument.createElementNS(HTML_NS, 'p');
+				elem.textContent = message;
+				_currentTooltipMessages.appendChild(elem);
+			}
+			_currentTooltipMessages.hidden = false;
+		}
+		else {
+			_currentTooltipMessages.hidden = true;
+		}
 	}
 	
 	
diff --git a/chrome/content/zotero/zoteroPane.xul b/chrome/content/zotero/zoteroPane.xul
index 4d20c0ad9a..c096d56392 100644
--- a/chrome/content/zotero/zoteroPane.xul
+++ b/chrome/content/zotero/zoteroPane.xul
@@ -214,6 +214,7 @@
 							<label id="zotero-tb-sync-label"/>
 							<label id="zotero-tb-sync-status" hidden="true"/>
 							<label id="zotero-tb-sync-last-sync"/>
+							<div xmlns="http://www.w3.org/1999/xhtml" class="sync-button-tooltip-messages"/>
 						</tooltip>
 					</toolbarbutton>
 				</hbox>
diff --git a/scss/_zotero-react-client.scss b/scss/_zotero-react-client.scss
index 54c74b6657..c99c1f191f 100644
--- a/scss/_zotero-react-client.scss
+++ b/scss/_zotero-react-client.scss
@@ -27,5 +27,6 @@
 @import "components/icons";
 @import "components/progressMeter";
 @import "components/search";
+@import "components/syncButtonTooltip";
 @import "components/tagsBox";
 @import "components/tagSelector";
diff --git a/scss/components/_syncButtonTooltip.scss b/scss/components/_syncButtonTooltip.scss
new file mode 100644
index 0000000000..b04d3ea2d2
--- /dev/null
+++ b/scss/components/_syncButtonTooltip.scss
@@ -0,0 +1,8 @@
+.sync-button-tooltip-messages {
+	max-width: 400px;
+	
+	p {
+		color: gray;
+		margin: 8px 6px;
+	}
+}
\ No newline at end of file
diff --git a/test/tests/syncRunnerTest.js b/test/tests/syncRunnerTest.js
index 1e9713addf..2dd4dbce0c 100644
--- a/test/tests/syncRunnerTest.js
+++ b/test/tests/syncRunnerTest.js
@@ -109,6 +109,63 @@ describe("Zotero.Sync.Runner", function () {
 		setHTTPResponse(server, baseURL, response, responses);
 	}
 	
+	function setDefaultResponses(options = {}) {
+		var target = options.target || 'users/1';
+		var headers = {
+			"Last-Modified-Version": options.libraryVersion || 5
+		};
+		var lastLibraryVersion = options.lastLibraryVersion || 4;
+		setResponse({
+			method: "GET",
+			url: `${target}/settings?since=${lastLibraryVersion}`,
+			status: 200,
+			headers,
+			json: {}
+		});
+		setResponse({
+			method: "GET",
+			url: `${target}/collections?format=versions&since=${lastLibraryVersion}`,
+			status: 200,
+			headers,
+			json: {}
+		});
+		setResponse({
+			method: "GET",
+			url: `${target}/searches?format=versions&since=${lastLibraryVersion}`,
+			status: 200,
+			headers,
+			json: {}
+		});
+		setResponse({
+			method: "GET",
+			url: `${target}/items/top?format=versions&since=${lastLibraryVersion}&includeTrashed=1`,
+			status: 200,
+			headers,
+			json: {}
+		});
+		setResponse({
+			method: "GET",
+			url: `${target}/items?format=versions&since=${lastLibraryVersion}&includeTrashed=1`,
+			status: 200,
+			headers,
+			json: {}
+		});
+		setResponse({
+			method: "GET",
+			url: `${target}/deleted?since=${lastLibraryVersion}`,
+			status: 200,
+			headers,
+			json: {}
+		});
+		setResponse({
+			method: "GET",
+			url: `${target}/fulltext?format=versions`,
+			status: 200,
+			headers,
+			json: {}
+		});
+	}
+	
 	
 	//
 	// Tests
@@ -993,28 +1050,18 @@ describe("Zotero.Sync.Runner", function () {
 			}
 		});
 		
-		it("should show the sync error icon on error", function* () {
+		it("should show the sync error icon on error", async function () {
 			let library = Zotero.Libraries.userLibrary;
-			library.libraryVersion = 5;
-			yield library.save();
+			library.libraryVersion = 1;
+			await library.save();
 			
 			setResponse('keyInfo.fullAccess');
 			setResponse('userGroups.groupVersionsEmpty');
-			// My Library
-			setResponse({
-				method: "GET",
-				url: "users/1/settings",
-				status: 200,
-				headers: {
-					"Last-Modified-Version": 5
-				},
-				json: {
-					INVALID: true // TODO: Find a cleaner error
-				}
-			});
+			
+			// No other responses, so settings response will be a 404
 			
 			spy = sinon.spy(runner, "updateIcons");
-			yield runner.sync();
+			await runner.sync();
 			assert.isTrue(spy.calledTwice);
 			assert.isArray(spy.args[1][0]);
 			assert.lengthOf(spy.args[1][0], 1);
@@ -1097,6 +1144,118 @@ describe("Zotero.Sync.Runner", function () {
 		});
 		
 		
+		it("should show an error for invalid My Library data", async function () {
+			let library = Zotero.Libraries.userLibrary;
+			library.libraryVersion = 1;
+			await library.save();
+			
+			var collection = await createDataObject('collection', { synced: true });
+			var json = collection.toResponseJSON();
+			json.version = json.data.version = 2;
+			json.data.INVALID = true;
+			
+			setResponse('keyInfo.fullAccess');
+			setResponse('userGroups.groupVersionsEmpty');
+			setDefaultResponses({
+				lastLibraryVersion: 1,
+				libraryVersion: 2
+			});
+			setResponse({
+				method: "GET",
+				url: "users/1/collections?format=versions&since=1",
+				status: 200,
+				headers: {
+					"Last-Modified-Version": 2
+				},
+				json: {
+					[json.key]: 2
+				}
+			});
+			setResponse({
+				method: "GET",
+				url: `users/1/collections?format=json&collectionKey=${json.key}`,
+				status: 200,
+				headers: {
+					"Last-Modified-Version": 2
+				},
+				json: [json]
+			});
+			
+			spy = sinon.spy(runner, "updateIcons");
+			await runner.sync();
+			assert.isTrue(spy.calledTwice);
+			assert.isArray(spy.args[1][0]);
+			assert.lengthOf(spy.args[1][0], 1);
+			// Not an instance of Error for some reason
+			var error = spy.args[1][0][0];
+			assert.equal(Object.getPrototypeOf(error).constructor.name, "Error");
+			assert.match(error.message, /^Some data in My Library/);
+		});
+		
+		
+		it("should show a warning in the sync button tooltip for invalid group data", async function () {
+			win = await loadZoteroPane();
+			var doc = win.document;
+			
+			// Create group with same id and version as groups response
+			var groupData = responses.groups.memberGroup;
+			var group = await createGroup({
+				id: groupData.json.id,
+				version: groupData.json.version
+			});
+			group.libraryVersion = 1;
+			await group.save();
+			
+			var collection = await createDataObject('collection', { synced: true });
+			var json = collection.toResponseJSON();
+			json.version = json.data.version = 2;
+			json.data.INVALID = true;
+			
+			var target = 'groups/' + group.id;
+			setResponse('keyInfo.fullAccess');
+			setResponse('userGroups.groupVersionsOnlyMemberGroup');
+			setResponse('groups.memberGroup');
+			setDefaultResponses({
+				target,
+				lastLibraryVersion: 1,
+				libraryVersion: 2
+			});
+			setResponse({
+				method: "GET",
+				url: target + '/collections?format=versions&since=1',
+				status: 200,
+				headers: {
+					"Last-Modified-Version": 2
+				},
+				json: {
+					[json.key]: 2
+				}
+			});
+			setResponse({
+				method: "GET",
+				url: target + `/collections?format=json&collectionKey=${json.key}`,
+				status: 200,
+				headers: {
+					"Last-Modified-Version": 2
+				},
+				json: [json]
+			});
+			
+			await runner.sync({ libraries: [group.libraryID] });
+			
+			assert.isTrue(doc.getElementById('zotero-tb-sync-error').hidden);
+			
+			// Fake what happens on button mouseover
+			var tooltip = doc.getElementById('zotero-tb-sync-tooltip');
+			runner.registerSyncStatus(tooltip);
+			
+			var html = doc.getElementById('zotero-tb-sync-tooltip').innerHTML;
+			assert.match(html, /Some data in .+\. Other data will continue to sync\./);
+			
+			runner.registerSyncStatus();
+		});
+		
+		
 		// TODO: Test multiple long tags and tags across libraries
 		describe("Long Tag Fixer", function () {
 			it("should split a tag", function* () {