Accidental? Fuck man, it's the most fun you'll have. Halo 3 in particular, driving a warthog off a cliff but getting out at the last second, throwing grenades at your teammates while they're in combat so the enemy gets the kill, or piloting a vehicle out of bounds so your passengers are killed by "The Guardians" is a pile of fun. The best is putting a sniper round through your teammate as soon as they've taken any damage from an enemy. It'll kill them outright, and the game will credit the kill to the enemy, because it gives you the benefit of the doubt.