User(s) blindly Check Out everything that modifies locally, without understanding why
This is my favorite. Every so often a user picks a layer in an assembly, changes and saves the status and then has hundreds of parts modified. So why not? Check them all out so everyone else on the program is looking at hundreds of out-of-date parts. I called it carpet bombing. I figured that carpet bombing was a flag for better training.
Intralink (RIP) and Windchill not automatically locking items on copying them out is the UI equivalent of staging bum fights. The development team has to know that of the tens of thousands of adding items to the workspaces for the 1% or less that are to be modified that the worst choice in a multiuser environment is the one that maximizes the chance for conflict between users, but it's the most fun.