It really ticks me off when people just enabled everything they download. It just leaves you asking "Why do you need 5 different rocket launchers, weighted cubes, assorted planes, lazers, and all this music in your Medieval DM?"
It just doesn't add up in my mind.
Another is when people start threatening you with crap for silly things like build trust or admin.
Some guy threatened to steal my GLaDOS for trust or something, I thought it was just funny that he thought he could even possibly get away with it.