When event_scheduler is set to DISABLED, event-related activities produce ER_EVENTS_DB_ERROR:

MariaDB [test]> show events;

ERROR 1577 (HY000): Cannot proceed because system tables used by Event Scheduler were found damaged at server start

At the same time, trying to change the value at runtime produces a different error:

MariaDB [test]> set global event_scheduler=on;

ERROR 1290 (HY000): The MariaDB server is running with the --event-scheduler=DISABLED or --skip-grant-tables option so it cannot execute this statement

This is confusing, especially when it comes to investigation of real-life problems.
If we can't distinguish real corruption from the user setting, which is probably the case, maybe it would be best to disallow user to set the option to DISABLED and keep it an internal-only value.