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 can only be performed on the Halo 3 level The Covenant. This glitch is a combination of three methods used to attain a second Arbiter.

When you have deactivated the third tower's barrier, you can run past the Flood (but kill them on higher difficulties) and go activate the elevator. When you come down, you will see Arbiter and his surviving Elites fighting the Flood. Kill the Flood, then kill Arbiter, and go to the entrance of the third tower. The second Arbiter will spawn with a Energy Sword. On higher difficulties, headshot the Human Combat Forms and stick the Carrier Forms or let the Arbiter handle it. When the first Arbiter and his Elites leaves the tower, go outside and you will see a Separatist Phantom. Flood Infection Forms should spawn. Kill them but don't go down to the Phantom. There is a loading point that will cause Arbiter to get on the Phantom. Destroy the Phantom with a Fuel Rod Gun without getting the loading point. Four shots to the Phantom's frontal bulbs or tails will destroy the Phantom on any difficulty. If Tilt is turned on, eight shots will destroy the Phantom. Now headshot the Sergeant (wait for a while on higher difficulties as a precaution) and then get the loading point. You will get a checkpoint and your betrayal count will be reset. Now headshot the Marine on the Mongoose. Killing the Sergeant and the Marine will make it more easier to get the Arbiters in the Warthog. Then kill the second Arbiter. Then kill the first Arbiter. It is necessary to kill them to ensure that the Arbiter operating the turret will actually shoot the weapon. Also, in the Citadel, it will ensure all Arbiters participate in the battle (on higher difficulties, you can only bring the Brute Chieftain to the third bridge). Now get in the Scorpion tank to get rid of the Marines and get out and get in the Warthog. The two Arbiters should get in the Warthog.

Drive the Warthog to the Hornets. It is possible to drive to the Hornets without killing all of the enemies; any enemy that didn't die will be instantly killed except for the first War Chieftain. On higher difficulties, try to ram into the Pilot that gets out of the Hornet. Never trust a Pilot to do your work for you; they will usually get the Hornet destroyed. Then get the other Pilot out and if you don't want him to take the Hornet, push him off the cliff, but don't melee him; if you melee him, he will attempt to ride in the Hornet. After you have killed the two Scarabs, the third Arbiter will be spawned. Get the two Arbiters in the Warthog to the Citadel; drive the Warthog to the Citadel. Now if you want the third Arbiter to respawn with a different weapon (he will always respawn with the Carbine if he runs into the Citadel as scripted), get the Arbiter on the Warthog turret out and push and melee him off the light bridge. Then get on the turret and the third Arbiter should get in the driver's seat. Then get the third Arbiter out and push and melee him off the light bridge. That should negate his scripted, designated course; he should now be staying still. Finally, get the Arbiter you didn't push off the light bridge out (he should be driving the Warthog) and push and melee him off the light bridge. Now use a heavy weapon (if Tilt is turned on) and sticky grenades (one sticky grenade will be sufficient unless Tilt is turned on) to destroy the Warthog and go down to the Arbiters, but don't just jump off the light bridge. You will either be affected by fall damage or you will die. Finally, give the Arbiters any weapons you want to give them and push and melee (one at a time) them into the Citadel.

After the Cortana Moment, just go and get the loading point; you do not have to kill the third Arbiter. After the cutscene, get to the third bridge. There will be four Arbiters. They will be standing still unless you choose not to kill the Brutes (on Easy difficulty) or the Brute Chieftain (on higher difficulties). Then the Arbiters will fight. To get the Arbiters to the end of the level, knock them (particularly the three killable ones) off the platform and activate the light bridge. After the cutscene, if you timed it correctly, you will still have four Arbiters and they will help you kill the Flood, especially on higher difficulties.

If you mess up, press the Xbox Guide button and sign out and sign in again and resume from the last checkpoint.

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.

The Four Arbiters Glitch works on The Covenant, when the Flood arrive on the Ark, since the game is scripted to spawn the second Arbiter when it cannot detect the first Arbiter. This is why when you choose not to destroy the Anti-air Wraiths, deactivate the third tower, and go outside, the second Arbiter will be spawned regardless of your choice.

Cloning the Arbiter by killing him prior to a cutscene is merely 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.