Overheat Grenade

The Overheat Grenade Glitch is a button combo.

Walkthrough
You must have a Plasma Pistol and at least one Plasma Grenade.

When you release a Plasma bolt after the pistol is charged and released, you can hit Y and L quickly after each other and the Grenade will fly without its throwing animation. It especially helps if you missed the plasma shot, because your opponent will most likely take cover behind a wall. After the grenade is thrown, there is a small time you cannot shoot, but doing a cancel reload (Y, Y) fixes that.

Note: This will not work on Halo 3.