I am sure this has been mentioned, but people who can't or won't handle a 4-way stop correctly. I HATE people who wave me through an intersection when they clearly had the right of way. I have seen more than 1 accident caused by the confusion of other drivers when one idiot is waving people through out of turn.
good catch, I forgot to mention that one. I get kind of indignant about it. I will not go. I turn my head and look away from them until they go.