Test on all browsers Log in as a user Add sleep(10) to lib/ajax/service.php right before the echo json_encode($responses); (to simulate a lot of work to do) Click rapidly through a few different pages, before the messaging and notification icons/panels get enough time to load Make sure you don't get an error pop up modal. On chrome/firefox check that you see the error in the console. No need to do this on multiple browsers Go to the dashboard Before it loads, click to go to site home (or anywhere else) Make sure no error shows After half a second, but still before the page loads, press stop on the browser (or press the hotkey... usually escape) Make sure no error pop up modal shows Remove the sleep(10) from lib/ajax/service.php Go to some AJAX-y part of moodle (e.g. Site administration > Plugins > Activity modules > External tool > Manage tools) Cause an error (e.g. Add a tool with the url http://lti.tools/test/tp.php and click the return "with an error" link) Make sure the error displays as an in-page notification Add a echo "." to lib/ajax/service.php Make sure you get a pop up error modal