What Is Your Biggest DMing Pet-Peeve?

What is something that players do in games that really grinds your gears as a DM?

Personally, it drives me crazy when players withhold information from me. Look guys, I know i'm controling the badguys, but i'm not your enemy! If you want to do something or make something work, talk to me! Trying to spring stuff on me that you've been holding onto doesn't make you clever, it just ends up making me grumpy, especially if it's not going to work!