Bouncing items glitch
From Halopedia, the Halo wiki
The bouncing items glitch is a Halo 2 glitch that causes some items, especially Plasma Grenades, to bounce when dropped on a moving elevator. This glitch can be seen in the Campaign level, The Arbiter. This also happens on the Halo: Combat Evolved levels Assault on the Control Room, 343 Guilty Spark and Two Betrayals.
As the elevator in The Arbiter goes down toward the room where you battle more Elites and Grunts, throw a Plasma Grenade upward. When it hits the floor of the moving elevator, it should bounce up and down without exploding. It will explode when the elevator stops.
Plasma Grenades do not explode until they "stick" to a valid target, such as a character or vehicle. They bounce off of all other surfaces. Throwing the Plasma Grenade upward in a descending elevator causes it to bounce almost infinitely, as the peak height of its bounces relative to the elevator's floor will never decrease until the elevator stops moving.