In a group I often practice with we have a little tradition where if somebody asks what team you're on during a team game, you can't lie to sneak up on them. You don't have to answer, although that would quickly give away your evil intentions, but you can't lie.
I am going to my very first event, Armageddon, next week. If during a great big team battle I see a shady character sneaking around behind me, can he lie to me if I ask what team he's on?
E d i t: Citing the articles of the Geneva Convention (I feel like such a dork), pretending to be an enemy is a war crime, so... I dunno. Not like that ever stopped anybody before I guess.