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 need it to drain the shields of three Arbiters. Continue to the third tower.

Deactivate the third tower. After the cutscene, you can escape the initial Flood and activate the elevator. When you get downstairs, the Arbiter and his Elites will be confronting the Flood. Kill the Arbiter and go to the entrance of the tower, ignoring the Flood. You should see a waypoint; this newly spawned Arbiter will have the Energy Sword as his primary weapon. This Arbiter is the same as the one spawned when you enter the third tower without destroying the Anti-air Wraith. Retreat back into the tower and destroy the Flood. When you have destroyed the Flood, the first Arbiter should get up.

Go outside. There will be a Separatist Phantom. You can destroy the Phantom with a Scorpion tank, melee the Plasma Cannon support platform off, kill the 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. It is possible to drive to the Hornets without killing any enemies, but the Wraith gunner will drain your shields. After you have killed the two Scarabs, the third Arbiter will be spawned.

Get the three Arbiters into the Citadel. Then melee the third Arbiter three times (five times with the Mythic Skull turned on) and headshot him as you get the loading point. After you have killed all enemies on the first and second bridge, go to the third bridge. When you get to the end of the bridge, there will be four Arbiters. Get the three Arbiters that can die in a cluster. Deploy the Power Drain and headshot the three Arbiters and activate the bridge.

You will have four Arbiters that you can bring to the end of the level. But they aren't exactly your friend anymore so hide behind something. When you see the yellow blips on the motion sensor or see the waypoint, come out of hiding. 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.