ONI: Sword Base Vacation
From Halopedia, the Halo wiki
The ONI: Sword Base Vacation is a glitch in the Halo: Reach campaign level ONI: Sword Base which allows the player to break the boundries of the playable area and explore the level at will. It requires two players, the Tilt and Cowbell skulls, a Concussion rifle, Armor Lock, and a Forklift.
The second forklift may be launched out of the map. This may prove useful for exploring.