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
Killing the Arbiter immediately before you activate the Cartographer will result in his duplication, though one of the Arbiters will leave as scripted. The Arbiter spawned outside the building at Mission Start and Rally Point Alpha is invincible. The Arbiter spawned inside the building can be killed.

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.

Four Arbiters
The Four Arbiters Glitch is a variant of the Two Arbiters Glitch that can only be exploited on The Covenant. The glitch can only be performed at Mission Start and Rally Point Alpha.

When you have killed the Anti-Air Wraith that the Loyalist Phantom drops on the island, kill all the Brutes and land your Hornet on the island. Pick up the Power Drain; you will deploy the equipment near the end of the Revelation chapter. Continue to the third tower.

When you can land the Hornet at the third tower, pick up a Beam Rifle. Deactivate the third tower as normal. After the cutscene, you can escape the initial Flood hostiles (though you should destroy them on higher difficulties) and activate the elevator. When you go downstairs, you will find the Arbiter and his surviving Elites being attacked by the Flood. Kill the Arbiter and go to the entrance of the tower, evading the Flood (though on higher difficulties, you should kill some of the Flood hostiles inside the tower). You should see a waypoint; this newly spawned Arbiter will have the Energy Sword as his primary weapon. Retreat back into the tower and destroy the Flood. When you have destroyed the Flood, the first Arbiter should get up.

To prevent the second Arbiter from being removed from the level, you can destroy the Phantom with a Scorpion tank, destroy or melee the Phantom Plasma Cannon support, or kill the second Arbiter. Get in a Warthog and get the two Arbiters to ride in the Warthog. Also, when you encounter the Prowler, you can hijack the vehicle/kill the riders and drive that one instead. Drive the Warthog/Prowler to the Hornets (you may need to get in a Scorpion or give the Arbiter a Rocket Launcher/Fuel Rod Gun if you are playing on a higher difficulty setting).

After you have killed the two Scarabs, the third Arbiter will be spawned. Now, you should get the three Arbiters into the Citadel. After the Cortana Moment, save and quit. Then resume the game and melee the third Arbiter three times (five times with the Mythic Skull turned on) and headshot him as you get the loading point.

From this point on, do not save and quit. After you have killed the enemies on the first bridge, do not go through the Forerunner door, but climb onto a supply case and jump on top of the structure. From here you can secure the second bridge with relative ease. Remember the Beam Rifle you picked up? You can easily snipe the Jump Pack Brutes and the Brute Captains, making crossing ths bridge significantly easier.

After you have killed all the Brutes on the second bridge, go to the third bridge. When you get to the end of the bridge, there will be four Arbiters. If there are four Arbiters, then save and quit. To bring them with you to the end of the level, first, move or melee the three Arbiters that can be killed in a cluster. You should block the invincible Arbiter with a Deployable Cover or move/melee him from you. Then headshot the three Arbiters and activate the bridge.

If you performed the glitch correctly, you should have four Arbiters correctly identified as enemies, so you should take cover behind something. When you see the yellow blips on the motion sensor or see the waypoint, come out of hiding. Now you will have four Arbiters helping you escape the Flood.

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 him prior to a cutscene is simply a manifestation of the Life After Death glitch.

Halo
By using a Spartan Laser with infinite ammo to kill the Arbiter immediately before the 343 Guilty Spark fight, players can create a "clone" of the Arbiter. Outside the Control Room, after the battle, one Arbiter will be standing around, and the other 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.