Life After Death
From Halopedia, the Halo wiki
Life After Death is a glitch present in Halo 3's Campaign. If an NPC is betrayed immediately before a cutscene that depicts their death, then after the cutscene, the NPC may still be alive.
The glitch does not affect the storyline.
Halo 3
Sergeant Johnson
On the level Halo (Level), when you arrive at the entrance to the Control Room, shoot Sergeant Johnson in the head to make him fall for 5 seconds. Enter the Control Room while Johnson is down. If done correctly, he will still be alive after the cutscene. However, after 3 seconds, a Flood Infection Form will jump onto him.
On higher difficulties, the Infection Form will vanish, unable to infect Johnson. He sometimes disappears as soon as you leave the Control Room's entrance door, though there is at least one recorded instance where a player brought Johnson all the way to the end of the level.
Notes
The Two Arbiters Glitch, when done on Halo, is performed identically, and in that instance may be the same glitch.
