Elevator lurkers.
You press the button on the elevator, and wait for the doors to open. There's someone in there, but they don't step out. Then, just as you move to get on the elevator, the moron comes to the conclusion that THIS is the floor number he pressed, and proceeds to walk off, always meeting you at the pinch-point of the door.
Look, the damn thing has a display of floor numbers, and beeps every time it passes a floor. Even with my expensive noise-cancelling headphones on, I know when my floor is coming up, and move toward the doors so I can step off immediately. People cannot be this stupid, can they?