Flyboy was a Halo 2 multiplayer glitch that could be performed on Coagulation. It required two players, an Energy Sword, one Warthog, and one Banshee. The glitch was fixed in Auto-Update 3; it can no longer be performed.
Have the player with an Energy Sword (Person A) sit in the passenger seat of a Warthog; the other player (Person B) should fly over them with a Banshee. As Person B flies over Person A, A should aim at them, attacking with the Energy Sword when their Reticle turns red. Person A will then zoom through the air after the Banshee for quite a few feet; more exactly, they will travel to where the Banshee was when they first sword-lunged out of the Warthog.
The glitch also worked with other vehicles and enemies that passed by Warthogs.
The glitch was similar in nature to Sword flying.