From Halopedia, the Halo wiki
Driverless Banshee is a glitch in the Halo 2 multiplayer level Waterworks. The glitch makes it possible for a pilot-less Banshee to fly temporarily, though if done correctly, it can result in being killed by The Guardians.
First, fly the Banshees to the top of the center part of the level, and then fly them through the top of the smashing block when it is up, to avoid getting killed by opposing players.
Next, make utterly sure that they are side by side, with their fronts facing different, angular directions. Hit one Banshee's wing, so it goes under the other Banshee's wing.
Then, get in one Banshee, and then the other, then the other, and so on, continuing to switch Banshees until both Banshees have the lights on their wings lit up. Fly one halfway up the block, still avoiding being squashed. The other Banshee should fly up too. Park the Banshee on top of the block, get out, and quickly hit the Banshee with the Rocket Launcher again.
One Banshee should get launched high up in the air, and the second one will do the same thing. Then, one Banshee should hopefully fly, then lose altitude and eventually fall down to the ground.