Using Onager early
From Halopedia, the Halo wiki
There are two methods to perform this glitch-
First, stand on the railing of a small set of stairs leading down to the landing pad at the very beginning of the section "Keyes". There should be a pipe to the top right, above you. Use the concussion rifle to launch yourself up by shooting at your feet, and land on the pipe. Then, launch yourself up again to the platform with the MAC gun. You can now operate and fire the MAC gun as normal and use it to shoot down the various Banshees and Phantoms flying around.
The second method involves jumping on a Phantom as it leaves after dropping troops, and using Armor lock as one hits the death barriers. Then, you will have to jump off and bounce off the rock wall. Finally, walk to the Onager. This also causes the player to leave the level's map.