Floating vehicles/player glitch
From Halopedia, the Halo wiki
In Halo 3 on the level: The Covenant, get to the part in the level where you destroy 2 Scarabs, destroy them and then, instead of following the Arbiter into the Ark's control room, take a vehicle for example a ghost or warthog (if any have survived) over the artificial cliff behind the control room. This can be a little difficult due to a ridge at the bottom of the upward slope, however, you can get over the ridge by using one of the sloping ramps attached to the side of the control room.
If you do manage to do this, instead of falling to your death, the vehicle will remain suspended in mid air. Additionally if you did this with a vehicle with no passengers, you can get out of the vehicle and you will remain floating in the air while the vehicle falls.
This will also work with the two flying vehicles (Hornet and Banshee), just let them settle on the invisible floor over the artificial cliff and get out.
To get out of this status, either kill yourself using a grenade against a wall (you are able to move slowly in any direction except up and down) or reset to a previous check point.
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