Setup (before the patch!) Clone a site (but don't install yet) and checkout to the version released before MDL-67494 (e.g. for master: " git checkout 95dd305cc6 "): Version Commit hash master 95dd305cc6 3.11 4652b66ee4 3.10 3799c7f535 3.9 f32754319e Proceed with the install now. Log in as admin Go to Site administration > Courses > Backups > General import defaults , enable Allow admin conflict resolution . Save the changes. Restore the backup-moodle2-course-5-testing_mdl-71156-20210409-1910.mbz that adds a course, with some enrolled users and groups. Note that action events, user and group overrides will be restored in the backup file. Create other events: Standard - user, site, category, course, group events Subscription - create a calendar subscription using the following iCal link: https://calendar.google.com/calendar/ical/en.usa%23holiday%40group.v.calendar.google.com/public/basic.ics Set the event type of these events from the subscription as Site events. Now checkout to the version with MDL-67494 code: Version Commit hash master 96b49ddc 3.11 f23ffe66 3.10 c6f3ecb4 3.9 329a5109 Upgrade your moodle instance again. Upgrade step and ad-hoc test (after the patch!) Check out the latest version for your moodle instance (e.g. for master " git checkout master ") Run upgrade. Make sure a new record has mdl_task_adhoc for \core\task\calendar_fix_orphaned_events adhoc task. Run the command php admin/cli/adhoc_task.php --execute Make sure the adhoc is executed and displays a summary with the number of affected events, similarly to this one (with different numbers, of course): Execute adhoc task: core\task\calendar_fix_orphaned_events ... started 17:05:53. Current memory use 16.6MB. Totals: 17 / 16 (total / wrong) - standards events: 5 / 4 (group: 1, course: 1, category: 1, site: 1) - subscription events: 0 / 0 - all good! - override events: 9 / 9 (quiz: 5, lesson: 4) - action events: 3 / 3 (quiz: 1, lesson: 2) - other events: 0 / 0 - all good!   Processing override events .. Finished!   Processing standard events .... Finished!   Processing action events Finished!   ... used 94 dbqueries ... used 0.15929698944092 seconds Adhoc task complete: core\task\calendar_fix_orphaned_events Run the command php admin/cli/fix_orphaned_calendar_events.php Make sure it displays 0 as the number of wrong events, eg Totals: 17 / 0 (total / wrong) Setup again (before the patch!) Repeat all the step in the first "Setup" section, creating a site with events and restoring the backup file. Before the patch! Testing ad-hoc task runtime (after the patch) Check out the latest version for your moodle instance. Open you config.php and add " $CFG->calendareventsmaxseconds = -1;"  and save. Run the upgrade. Run the command: php admin/cli/adhoc_task.php --execute Make sure that after it started processing of events, it stops the execution showing the pending message and on which type of event, eg: Remaining EVENTTYPE events pending Run the same command again and confirm it display the pending message until it finishes fixing all events. CLI script Restore the backup with the calendar events. On your database, run the UPDATE query to simulate the existence of problematic events. UPDATE mdl_event SET userid = 0 WHERE eventtype <> 'user' OR priority <> 0 On your terminal, run the following command: php admin/cli/fix_orphaned_calendar_events.php Make sure a summary with the number of affected events is displayed, similarly to this one (with different numbers, of course). e.g. in master: == Checking the site status ==   This site has executed the problematic upgrade step 2021052500.65 present in 4.0dev. This site has executed the fix upgrade step 2021052500.85 present in 4.0dev.   == Checking the calendar events status ==   Totals: 17 / 16 (total / wrong) - standards events: 5 / 4 (group: 1, course: 1, category: 1, site: 1) - subscription events: 0 / 0 - all good! - override events: 9 / 9 (lesson: 4, quiz: 5) - action events: 3 / 3 (lesson: 2, quiz: 1) - other events: 0 / 0 - all good!   This site NEEDS to run the calendar events fix! To fix the calendar events, re-run this script with the --fix option. Run the command again, with the --fix option this time. It will display a confirmation message, type Y to start the execution. Make sure the script fixes those orphaned events and display a summary like this one: == Fixing as many as possible calendar events ==   Processing override events .. Finished!   Processing standard events .... Finished!   Processing action events Finished!   == Checking the calendar events status (after fix) ==   Totals: 17 / 0 (total / wrong) - standards events: 5 / 0 - all good! - subscription events: 0 / 0 - all good! - override events: 9 / 0 - all good! - action events: 3 / 0 - all good! - other events: 0 / 0 - all good! If you compare the summary before/after you can see it has 0 calendar events showing as wrong.