Come on, be real.
If someone had a password and wanted to cause trouble do you think they would randomly pick one a account once a week and mess it up?
Actually what happens more precisely is.
User suddenly cannot Login.
All Posts by that user show as unregistered.
Checking member list that user is still their and their member file is their. Clear caches does nothing.
When in this state I reindex and then the member file is gone and the user no longer exists.
To fix I add the user again, rename the member file to the old ID and reindex.
