To test this we need to simulate a high concurrency for a 'core_cron' lock. The easiest way to do that is to hack the code. Install testtasks: git clone https://github.com/catalyst/moodle-tool_testtasks admin/tool/testtasks Queue a number of tasks: php admin/tool/testtasks/cli/queue_adhoc_tasks.php -d=5 -n=50 Open lib/classes/task/manager.php in your editor, find the get_next_adhoc_task function and just before the foreach ($records as $record) line add following: sleep(20); Open two terminal windows In the first run: php admin/cli/cron.php Wait a couple of seconds and then, in the second terminal, run: php admin/cli/cron.php Confirm , that `locktimeout` exception is thrown by the second cron: !!! The operation timed out while waiting for a lock. !!! Confirm , that unreleased lock exception has not been thrown: Uncaught coding_exception: Coding error detected, it must be fixed by a programmer: A lock was created but not released at... Run adhoc_task.php from the second terminal: php admin/tool/task/cli/adhoc_task.php --execute ##  Confirm , that `locktimeout` exception is thrown by adhoc_task.php: !!! The operation timed out while waiting for a lock. !!! Confirm , that unreleased lock exception has not been thrown: