Nightfall forklift glitch
From Halopedia, the Halo wiki
The Nightfall forklift glitch, or simply forklift glitch, is a glitch which uses a forklift on the Halo: Reach level Nightfall and allows the player to go through the gate near the stealth pylon. It is especialy useful when trying to complete this level on Legendary.
Once the forklift is secured, it must be driven up to the gate, with forklift's left side as close and as square as possible to it. The player must then get out of the vehicle, and they will end up on the other side. It may take a few tries, as it must be positioned perfectly. After performing this glitch, the enemies on the rest of the level will not spawn.