From Halopedia, the Halo wiki
Have one player (Person A) enter Edit Mode, spawn a Mongoose, set it down, and exit Edit Mode. Person A should then enter the driver's seat, and another person (Person B) should enter the passenger seat. Person A should then get off the vehicle, return to Edit Mode, and pick it up.
Person A should now maneuver the Mongoose outside of the map, at which point Person B can dismount the vehicle and end up beyond the level boundaries. Person B can explore, but should be careful to avoid kill barriers, assuming that they weren't killed by such a barrier the moment they exited the Mongoose.
The glitch can also be done with Person B riding on a...