Using Onager early
From Halopedia, the Halo wiki
On the level The Pillar of Autumn in Halo:Reach, it is possible to board and fire the Onager before you are supposed to. In order to perform the glitch, you must have a concussion rifle and a drop shield, and have the Tilt skull enabled.
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.
This YouTube video shows how to perform the glitch: [1]
