Halo 3 invincibility glitch
From Halopedia, the Halo wiki
The Invincibility Glitch can cause the user to become invincible, hence the name. To perform the glitch, the host needs to be in Player Form while the other being in Monitor Mode. With both simultaneously picking up the weapon, the host bi-ped will have its 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.
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Another way to become "invincible" in Halo 3 is to go to Avalanche in Forge mode, spawn a closed container. Now put a turret up against the side of it, then get on the turret, look up and go into edit mode, now you should be inside the box. You can shoot out of the box and kill people but you cannot be injured or killed.
