Halo 3 invincibility glitch
From Halopedia, the Halo wiki
The invincibility glitch can cause the user to become invincible, hence the name.
Requirements[edit]
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.
Walkthrough[edit]
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.