Life After Death

Life After Death is a glitch present in Halo 3's Campaign. If an Non-Playable Character (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.

The Ark
Start the level at Rally Point Bravo. Kill the Arbiter and take his Carbine. The Arbiter should rise up and pick up his Energy Sword. If the Thunderstorm skull is not turned on, pick up a Deployable Cover. When you have reached the Cartographer room, you should melee the Arbiter to the activation panel and to the edge and headshot him and activate the Cartographer. After the cutscene, you should see the first Arbiter fall downstairs and rise up; this Arbiter will not have the waypoint. The second, newly-spawned invincible Arbiter will hijack the Banshee as scripted and be removed from the level. After you have killed all the enemies upstairs, go downstairs and go to the lobby and use the Deployable Cover to block the door. The Arbiter will kill the Brute Chieftain and then he will attempt suicidal death charges and attack the Jump Pack Brutes. You can melee the shield and exit the lobby when the Arbiter attacks the Jump Pack Brutes. Consider deploying Auto-Turrets before activating the Cartographer to aid the Arbiter.

If the Thunderstorm skull is turned on, Deployable Covers will not be available and the Arbiter will kill the Brute Chieftain, Brute Stalkers, and the Jackals. When you have killed some of the Jump Pack Brutes, the Arbiter can leave the lobby, but you must melee him.

Halo (Arbiter)
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 (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.

Explanation
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. How the glitch prevents their removal, however, is unknown.

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. Killing such an NPC will prevent their removal, though the reasons for this are unknown. When the game re-adds him later, it does not check to see if the previous NPC was successfully removed.

This, then, is the exact same glitch as the Three Johnsons glitch and the Two Arbiters Glitch.