I administered a VBulletin forum, and naturally, we installed all sorts of gewgaws onto it, including an "arcade" where people could play games, share high scores, etc.
This arcade, somehow, came with its own built-in comment system, one where users could somehow register without registering for a proper VBulletin user account on our instance, and thus without admins being notified.
One day, we discovered this whole underbelly community that had apparently been thriving under our metaphorical floorboards, and promptly evicted them. In hindsight, I probably should have found some way to let them stick around, but recently several things had happened that hardened our stance to any sort of un-wanted users.
