Life After Death
Life After Death is a glitch present in Halo 3's Campaign. If an NPC is betrayed immediately before a cutscene that depicts their departure or death, the NPC will still be alive and in the level after the cutscene. The glitch was named for its first application: "preventing" the death of Sergeant Johnson.
The glitch does not affect the storyline.
Start the level at Rally Point Bravo. Kill Arbiter and move his Carbine away from him. Make sure Arbiter picks up his Energy Sword, not a Plasma Pistol. If the Thunderstorm skull is not turned on, pick up a Deployable Cover. You can pick up a Deployable Cover in the room with the eating Brutes; the Brute Minors will usually drop a Deployable Cover when they die. When you have reached the Cartographer room, melee the Arbiter to the activation panel and position him so that when you headshot him, he will fall to the lower level. Then headshot him and activate the Cartographer. After the cutscene, you should see the first Arbiter fall to the lower level and rise up; this Arbiter will not have the waypoint. Make sure he has his Energy Sword. The second, newly-spawned and invincible Arbiter will hijack the Banshee as scripted and be removed from the level. After you have killed all the enemies on your level, go downstairs, go to the lobby, and use the Deployable Cover to block the door after the Chieftain has left the lobby. Arbiter will kill the Brute Chieftain and then he will attack the Jump Pack Brutes. You can drain the Deployable Cover and exit the lobby when Arbiter attacks the Jump Pack Brutes. Consider deploying Auto-Turrets before activating the Cartographer to aid Arbiter.
If the Thunderstorm skull is turned on, Deployable Covers will not be available and Arbiter will merely kill the Brute Chieftain, Brute Stalkers, and Jackals and remain in the lobby. When you have killed some of the Jump Pack Brutes, the Arbiter can leave the lobby, but you must melee him.
The first step requires you to be able to kill the Arbiter right before you enter the room where the battle with 343 Guilty Spark is fought. In order to achieve this, you will have to perform the Spartan Laser Glitch, and it is best to try it on Easy.
Once your Spartan Laser has been glitched into having infinite ammo, go to the door immediately before the fight with 343 Guilty Spark, but don't enter. Shoot the Arbiter with the Spartan Laser until he dies. Quickly after that, go through the door and initiate the cutscene. Once you finish the fight with 343 Guilty Spark, go outside and there will be two Arbiters. One will be standing, looking at you, and another will already be attacking the awaiting Flood; the latter has the waypoint above his head. Once near the end of the hallway, both Arbiters will stay back.
Halo (Sergeant Johnson)
On the level Halo, 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 Pod infector will jump onto him. If the player is able to save Johnson from the Pod infectors, kill Johnson twice to get his Spartan Laser with unlimited charge.
On higher difficulties, the Pod infectors will vanish, unable to infect Johnson. He sometimes disappears as soon as you leave the Control Room's entrance door, though there are at least two recorded instances where a player brought Johnson all the way to the end of the level.
It is known that, under normal circumstances, the affected characters are removed from the level after the affected cutscenes; on The Ark, the Arbiter departs in a Banshee, and on Halo, Sergeant Johnson dies. The glitch prevents them from being removed from the level.
When proceeding through these levels normally, important NPCs, such as Johnson and the Arbiter, are deleted from and re-added to the levels several times. Each time an important NPC gets separated from the player, they are removed from the map. Killing such an NPC will trick the game engine and prevent the NPC's removal. When the game spawns a new copy of the NPC later on (when the player is reunited with that character), it does not check to see if the previous copy was successfully removed.
This, then, is the exact same glitch as the Three Johnsons glitch and the Two Arbiters Glitch.