Open the 'Network' tab of your browser Navigate to Navigation -> My profile -> My private files Click the 'Add' button to load the File picker Confirm that the call to repository_ajax.php had a Type of 'application/json' Close the file picker Navigate to Administration -> Site administration -> Users -> Permissions -> Assign system roles Select a role Enter some content into the 'Search' box Confirm that the call to /user/selector/search.php had a Type of 'application/json' Edit /user/selector/search.php Add a call to print_error('foo'); somewhere in the output Enter some new content into the search box (no need to reload the page) Confirm that the AjaxException dialogue was shown complete with your message and stack trace throw a new moodle_exception (or similar) Enter some new content into the search box (no need to reload the page) Confirm that the Exception dialogue was shown complete with your message and stack trace