6. Phantom Gunfire
It's one of the most simple and childish ones possible, but at least it isn't
that harmful (at least compared to some of the other ones on here). Team killing is problematic because it'll most often get you booted from a server, but what about surreptitiously shooting your team-mates in the back when they're not looking, and then slyly turning in the other direction when they look around to find the shooter? They'll think an opponent is in the vicinity, and if you're lucky, you might even manage to drive them a little bit nuts. It's also fun if you can get the cover of enemy gunfire; get close to the enemy stronghold, make sure no other team-mates can see what you're doing, and start at it; if you're lucky, you might even soften them up for the "real" enemy, who will manage to mow them down with probably a single shot or two. Ok, so it's not that harmless, and in fact, it's pretty cruel too, because there's not much the victim can do about it, short of killing you, which of course allows you to troll even harder by getting them booted from the server.