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.

The Ark
Start the level at Rally Point Bravo. When you have reached the Cartographer room, you should melee the Arbiter off the platform and activate the Cartographer. After the cutscene, you should see the first Arbiter rise up in Theater; this Arbiter will have the waypoint over his head until the second is removed from the level. The second, newly-created Arbiter will hijack the Banshee as scripted and be removed from the level.

The first Arbiter will lose the waypoint, but he can reach the end of the level. After you have killed all the enemies upstairs, go downstairs and activate a loading point, but don't go through the door. Go back upstairs and the first Arbiter will go into the lobby and confront the enemies downstairs, including the Brute Chieftain, Jackals, and Brute Stalkers. However, he will remain in the lobby overseeing the extraction point until most of the Jump Pack Brutes are killed. Then he can be pushed to the end of the level.

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.