Destroyed vehicles glitch
From Halopedia, the Halo wiki
The Destroyed Vehicles Glitch is a glitch present in Halo 3. The glitch allows players to enter a destroyed vehicle, though the vehicle cannot be driven. This glitch can also be performed in Halo: Reach.
Walkthrough
Start by finding a vehicle (preferably a Banshee) and obtaining a Spartan Laser. Stand next to the vehicle (your body must be touching the vehicle). Press RT, then RB to enter the vehicle. Next, charge the Spartan Laser and just as you fire, enter the vehicle.
Another way to get inside a destroyed vehicle is to have someone damage the vehicle with a Fragmentation Grenade or two. The vehicle should be damaged enough that it will explode if one of its passengers has been stuck with a Plasma Grenade. One player (Person A) should stand next to the vehicle, while the other (Person B) should wait. Person A should enter the vehicle, and Person B should proceed to stick them and board the vehicle.
A variation on that method uses an Overshield and a Missile Pod. Person A would have the Overshield, and Person B would fire upon them while they enter (instead of throwing a grenade).
Tactical advantage
Forcing someone else to use this glitch can give you a tactical advantage over a boarder. If you get boarded while driving a Banshee, quickly ram into the nearest object. If you do this correctly, you have a chance of surviving the "accident" while your assailant is trapped inside the remains of the Banshee. Then lob a grenade or melee him to finish the job.
Additional information
This has been confirmed to work with the Banshee, Hornet, Mongoose, Warthog, Brute Chopper and Ghost. It does not work with other vehicles because boarding them does not result in the attacker taking a seat.
A player riding a destroyed vehicle will occasionally respawn. This happens when the vehicle's wreckage is removed from the map after a certain amount of time.
Using this glitch with a Banshee on Valhalla's river will create a raft-like experience, with the player floating downstream. If the Banshee is flipped, the player will not be ejected.
If done using a Ghost, there is a chance that the Ghost will explode harmlessly, pushing the player out of the wreckage.
