Halo 3 invincibility glitch
From Halopedia, the Halo wiki
The Invincibility Glitch can cause the user to become invincible, hence the name.
A Forge game with two players, the host in Monitor mode, the other in player mode. Also, any weapon that requires two hands to hold.
To perform the glitch, both players must simultaneously pick up the weapon, the host bi-ped will have his/her arms twisted at the back, with the weapon being controlled by the Monitor. By deleting the weapon or having the player switching weapons, the bi-ped will return to normal, however, with one arm disappearing. Through this method, the player will become invincible.
The only melee that can kill you is a Gravity Hammer blast, which can take up to two blasts to kill. You can still die from explosions and getting rammed by a vehicle. Though, when they die from being meleed by a Gravity Hammer, the game will lag for the rest of the game. Once you leave Forge, everything will be normal again.