I’ll tell you mine. It’s when people obliviously block an entrance or an exit, selfishly standing there without any sign of future motion, when it would be obvious even to a sight-impaired gnat that that streams of others are desperately trying to get in or out. This behavior is not only scarily dangerous, it’s incredibly piggish and obstinate, and I always fantasize about shoving these oafs to the ground and breezing over their bodies–but they’re generally very large.
Runner up: People who perversely push “Close” in an elevator when they see that you’re coming for it.
Second runner up: People who glance around the room for someone better as they’re talking to you.