The offboarding checklist is not enough. You should have regular cleanup procedures which synchronize accesses with the main source of truth as well. There are

The offboarding checklist is not enough. You should have regular cleanup procedures which synchronize accesses with the main source of truth as well. There are too many exceptions and just human errors for a time constrained checklist execution to be enough. Checklist is a 90% solution which might help here. Or it might have been the case where the checklist failed.