1. In any standard plugin in lib.php add the "after_config" callback. For example, add this code to blocks/online_users/lib.php : function block_online_users_after_config() { global $SITE; $SITE->fullname = 'hey'; } 2. Purge caches 3. Open the site homepage and make sure that the site name is displayed as 'hey' now (you may need to refresh the page couple of times). 4. Go back to the lib.php file you modified and instead of 'hey' write hey() (this will be a call to a non-existing function) $SITE->fullname = hey(); 5. Refresh the site homepage (may need several refreshes), and make sure that the normal site name is back and debugging message is displayed about an exception in callback. Before the patch your moodle site will be stuck on exception page here 6. Remove the function completely 7. Again, refresh the site homepage. Make sure there is a debugging message (it may be hidden under the menu bar) but site is still usable 8. Purge caches 9. Make sure site works as normal now without any debugging messages