Having your laptop die during a critical part of the semester when your renewed warranty has expired. That means spending money I don't have on a new one, losing recent schoolwork I was working on, plus I'm now going to be forced to switch to Vista. It's all giving me a big headache. *grumble*
Also, I assign people who stand chatting with each other in the middle of a path and don't move even when you say "excuse me."