Double Warthog driving glitch
From Halopedia, the Halo wiki
This is a very interesting glitch. First you park two Warthogs, front end parallel to the back end, side by side. So that the driver seat of both Warthogs are right next to each other. Then, you sit in both seats lots of times, which makes the computer think that you are in both Warthogs. Next, move one Warthog and the empty Warthog will go in the same direction. You can repeat the trick with other Warthogs, Scorpions, Ghosts, even Banshees and turrets. If you are in the driver's seat of a vehicle with driver fireable weapons i.e. the Ghost, simply pull the right trigger to fire off your guns. Also note that this trick still works if you get out of the vehicle (i.e. you work two Warthogs and then get out, the Warthogs will move with you).
This can also be done to take control of multiple vehicles by simply driving up to another Warthog, Ghost, Scorpion, etc. and repeating the process until you have control of three vehicles. Note that this can cause serious lag, and if you are hosting a server and someone is using remote control, you will not see it.