Two Arbiters glitch

The Two Arbiters Glitch is a glitch that can be exploited in three Halo 3 Campaign levels: The Ark, The Covenant, and Halo. The glitch, which can only be performed in single-player, causes the Arbiter to be duplicated for a portion of the level.

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 Chieftan, Jackals, and Stalkers. However, he will remain in the lobby overseeing the extraction point until the majority of Jump Pack Brutes are killed. Then he can be pushed to the end of the level.

Explanation
The Arbiter spawned outside the building at Mission Start and Rally Point Alpha is invincible. The Arbiter spawned inside the building can be killed.

Cloning the Arbiter by killing the Arbiter prior to a cutscene is simply a manifestation of the Life After Death glitch.

The Covenant
When you have de-activated the third tower, get in the tank before the Arbiter gets in the Phantom and blow it up while he's in it. The Arbiter won't leave the level like he's scripted to, and after the second comes down from the Phantom, it won't have a waypoint above his head. The second one may be teleported when you get to certain parts in the Citadel.

Another method simply requires that you kill the Arbiter just as Truth starts speaking, immediately before the cutscene when you enter the Citadel after defeating the two Scarabs.

Explanation
The glitch works on the Covenant because when the Phantom begins to engage its Active Camouflage, all of the Elites on board are removed from the level, as can be seen by players who actually get on the Phantom with the Elites. By destroying the Phantom, a player performing this glitch stops the Phantom from engaging Active Camouflage, which prevents the Arbiter and the Elites from being removed from the level. Therefore, the Arbiter remains with you. As the game is unaware of the Phantom's destruction and this Arbiter's continued existence, it spawns another Arbiter the next time the Phantom shows up.

Cloning the Arbiter by killing the Arbiter prior to a cutscene is simply a manifestation of the Life After Death glitch.

Halo
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 do 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.

Explanation
Cloning the Arbiter by killing the Arbiter prior to a cutscene is simply a manifestation of the Life After Death glitch.

Three Arbiters Variant
The Three Arbiters Glitch is a variant glitch that can only be exploited on The Covenant. The glitch causes the player to receive an additional Arbiter during the Flood attack outside the third tower.

The Covenant
Start at Rally Point Alpha. When you have killed the Anti-Air Wraith on the isolated island, kill all the Brutes and land your aircraft. Pick up a Deployable Cover. You will be using this to prevent the Elites from helping the Arbiter when he is attacked by the Flood. Now after you have killed all enemies downstairs, do not activate the elevator, but place the Deployable Cover in front of the door the Hunters were guarding. An alternate method is to kill all the Elites. Now activate the elevator and deactivate the third tower. After the cutscene, you can run past the initial Flood enemies (you should kill them on higher difficulties) and activate the elevator. When you go downstairs, you will find the Arbiter fighting the Flood alone. Kill this Arbiter and run to the entrance of the tower, ignoring the Flood (on higher difficulties, you should kill some of the Flood enemies inside the tower). You should see a waypoint; this second Arbiter will have the Energy Sword as his primary weapon. This is likely the same Arbiter that is spawned when the player decides not to destroy the Anti-Air Wraiths, but proceeds into the tower and then destroys the Wraiths. Then retreat back into the tower and kill the Flood. When you have killed all the enemies, the first Arbiter should rise up. To prevent the second Arbiter from being removed from the level, you can destroy the Phantom, kill an Elite operator, or kill the second Arbiter. Get in a Warthog and get the two Arbiters to ride in the Warthog. Drive the Warthog to the Hornets (you may need to get in a Scorpion tank or give the Arbiter a rocket launcher if you are playing on a higher difficulty setting). After you have killed the Scarabs, the third Arbiter will be spawned.