From Halopedia, the Halo wiki
Flipping Warthog is a glitch from the Halo 2 campaign level, Quarantine Zone. This glitch uses the doors on the level to make the Warthog flip end over end and, if done right, will make the player in the Warthog as well as the warthog itself fly a long way.
Begin the level Quarantine Zone, and continue past the first fight to the room with the Flood fighting the Sentinels trying to contain it. Once there, run forward to under the table like structure behind the wall when you first come in. There is a Warthog under it. Claim the Warthog and go back to the door you came in from. From there, aim straight at the door, and back up until it stops. Now go full throttle ahead trying to ram the door before it opens. You will hit it and, when the door opens, you will be propelled into the air quite a ways.
This glitch can also be performed with a Ghost. Simply steal the Ghost from your allies in the beginning and boost until you reach the door. Now go through it and boost at the door. The second you hit the door, press the spacebar on PC or A on xbox once and you will maximize flight height.
The Warthog can go farther, but the Ghost will go higher.