Dual Melee

The Dual Melee is a glitch that happens in Halo 2.

Walkthrough
In Halo 2, when wielding a Plasma Pistol, press the X and B buttons repeatedly. This will cause the Plasma Pistol to produce its normal melee animation, where the Master Chief uses his fist, and then another where he hits with the gun itself. This can be hard to see at first, since the animations occasionally "go through" one another, but it is still there.

It is unknown if this is a hidden animation left by Bungie, or a bizarre glitch, though it is most likely the latter.

If done using an Elite character model, the Elite's mandibles will open completely. You can see this in The Codex and This Spartan Life machinimas; the actors use the glitch for theatrical reasons.