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
On The Covenant, it is possible to get four Arbiters.

Start at Rally Point Alpha. When you have killed the Anti-Air Wraith on the isolated island, kill all of the nearby Brutes and land your Hornet on the island. Pick up a Deployable Cover; you will be using this to prevent the Elites from helping the Arbiter when he is respawned at the door, engaging the Flood. Continue to the third tower.

After you have killed all of the enemies downstairs, do not activate the elevator. Place the Deployable Cover in front of the door the Hunters were guarding or kill all of the Elites. Activate the elevator and deactivate the third tower as normal. After the cutscene, you can run past the initial Flood enemies (though 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 (though 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. Retreat back into the tower and kill the Flood.

When you have killed all of the enemies, the first Arbiter should rise up. To prevent the second Arbiter from being removed from the level, you can destroy the Phantom, destroy the Plasma Cannon support platform, 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 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 if you are playing on a higher difficulty setting). After you have killed the Scarabs, the third Arbiter will be spawned.

Next, you should get the three Arbiters into the Citadel. After the Cortana Moment, melee the third Arbiter three times (five times with the Mythic Skull turned on) and stick him with a Spike Grenade or headshot him with a Battle Rifle). Then get a loading point and kill all the enemies on the first bridge. You should headshot the Grunts and use a Beam Rifle or use the Plasma Pistol/Battle Rifle Combo on the Brute Captain. Frag grenades work well on the Jackals. Be wary of the Grunts when the Catch skull is enabled. After you have killed all the enemies, do not go through the door, but climb onto the supply case on the left or the right and jump onto the structure above the door. Here you can snipe the Brute Captains near the Chieftan and the Jump Pack Brutes. After you have killed all the enemies on the second bridge, go through the door to the third bridge. When you get to the end of the bridge, there will be four Arbiters.

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.