Infected Brute Chieftain

The Flood Brute Chieftain is a glitch that can be found on the Halo 3 level,.

Walkthrough
It can be done by getting a Flood Infection Form onto a Brute Chieftain. These two do not normally exist in the same area. The only way to get them together is to kill all your allies outside the third tower in the level, while leaving some Flood Carrier Forms alive. Kill the Carriers and let the Infection Forms chase you up into the snow area. Kill the War Chieftain there and let the Flood take him over.

After being infected, the Chieftain's appearance does not change except for some Flood infection on his back. He retains all his normal combat abilities, including vehicle boarding. His shields stay down. The infected Chieftain attacks both the player and the Covenant nearby. The Chieftain is vulnerable to headshots and assassination. Surprisingly, the Chieftain can pick up any weapon.

This glitch can also effect Elites and Marines but you must whack the dead body all the way from the 3rd tower to the hidden loading point.

Explanation
The Flood Brute may be an old version of combat forms created in-game. The Flood would not totally change the host's body immediately after infection. This idea was cut from the game, possibly because the combat forms looked too similar to the original enemy. The glitch occurs on The Covenant because there is a hidden loading point which removes all of the Flood from the level. When an Infection Form is in the process of transforming the dead Brute into a combat form and the player triggers the loading point, the Flood Infection Form is removed from the level, but ultimately resurrects the Brute, albeit his AI allegiance has been changed from the Covenant to the Gravemind.