A cheat in Halo 2 that exploits the Xbox Live
network code to make a player invisible and invulnerable, allowing him to capture a flag in 15 to 20 seconds. It involves getting in and out of the passenger side of a Warthog
repeatedly, then having a partner drive it to the base, at which point you take the flag, then take one step, which teleports you back to your base for the cap.
On a side note, Bungie
has made dummying a bannable offense.