Suspended dropships
From Halopedia, the Halo wiki
This glitch only works on Halo: Combat Evolved.
In the level Halo, there's a script glitch that affects the Covenant dropship that appears in the valley with the partially buried structure. It drops off some Elites and Grunts next to the crashed lifepod near one of the entrances to the valley. However, if you arrange to be standing on the high ground just inside the other entrance when the dropship arrives, it will hover indefinitely over the lifeboat until you leave the area (either deeper into the valley or out of it). During this interlude, the Elites riding in the starboard personnel bay of the dropship are immobile and utterly defenseless against sniping or embrace firing.