well ive known about this one forever so here it is.
set team death match with friendly fire on and hovercrafts on.
Go to desert with friendly fire on.
then get in a hovercraft and DON'T MOVE IT AT ALL!!!!
then get someone to shoot you in the head and kill you.
then have the person who shot you get in then right out of the hovercraft.
if done correctly you should be able to walk on air.
to get down roll against a wall.
NOTE** I also know how to get in the respawn room using this glitch.