Shielded dead Elite
From Halopedia, the Halo wiki
The shielded dead Elite glitch is a phenomenon that occurs in Halo: Combat Evolved and, when lag is involved, in Halo 3's multiplayer. It can cause the corpse of an Elite to still have an active personal energy shield despite the shield's deactivation typically being a prerequisite to them dying at all.
Halo: Combat Evolved
To perform this, find any Elite, in any level, on any difficulty. Without inflicting any damage on the Elite, assassinate it. Then, shoot his corpse. It should flicker, as though the Elite's shield were still functioning. This works with all types of Elites, and even Zealots.
The easiest way to see this glitch is to start Assault on the Control Room, and play until you get to the bridge. There is an Elite on the lower level (usually a Major). Simply catch him off guard, assassinate him, and shoot his corpse to see the glitch.