Perpendicular Banshee fire
From Halopedia, the Halo wiki
Perpendicular Banshee fire is a glitch in Halo: Combat Evolved. Point the Banshee so that it only goes straight up, then fire its twin plasma cannons or fuel rod cannon. Notice that the shots go perpendicular to the direction your Banshee is going, not forward like it usually does.
On the level Two Betrayals when you hop into the Banshee and fly back into the control room. Nose-dive into the large pit until you hit the bottom. Then get the Banshee to look directly up, when you start flying upward your fire should go to the side. When you get out of the pit, however, the glitch will stop and the Banshee will fire normally.
It also works in Halo: Combat Evolved for PC, in any Multiplayer map that supports the Banshee. Just fly up until you reach an invisible wall and fire the Banshees guns.