From Halopedia, the Halo wiki
First, grab a Banshee and place it near a Scorpion. Get out of the Banshee and ride the tank. Using the Scorpion, push the Banshee, so that one of its Gravity Pods is wedged into a crevice on a wall. The tip of the Banshee's wing should be stuck to the cliff wall, preventing it from taking off. The Banshee should now be almost completely immobile and should not move, even if players try to flip it or throw grenades at it.
Another way to make an immobile Banshee is to fly it over a wall, then dropping it so that its opening will "bite" the wall. Banshee strafing runs on the Blood Gulch bases often result in entrapment in the crenelations of the bases. Overzealous players who attempt to splatter players atop the base may find themselves lodged in one of the crenelations or the teleporter hub. In this instance, it is sometimes possible to dislodge the Banshee, but is oftentimes impractical as the Banshee's position leaves it susceptible to a grenade.
Also, in Blood Gulch, the banshee can be landed in a tree that is present at both bases. If done correctly, it will get stuck in the tree and explosions will not be able to dislodge it. It should shake the whole time and not respawn. The inability to respawn is because the Banshee is always in motion, and therefore never at rest.
Playing with the immobile Banshee can be dangerous. If the player attempts to flip it while near the wall, they may be killed.
This glitch requires a little luck and a lot of patience, but it is possible.
- The immobile Banshee glitch is one of many glitches that exploit holes in the physics engine's collision-scanning methods.