Testing vs prod bugs are always FUN.

In my first job we had warehouse management system, and for testing new versions we allowed users to log-in to test environment.

Some employees didn't knew they were supposed to only log in to prod and happily worked in their warehouse accepting deliveries, stocktaking, moving stuff in real world using test db instead of the prod one. We only realized when they moved so much stuff that the inconsistencies db vs reality triggered alarms.