Two Arbiters glitch

The Two Arbiters Glitch is a glitch that can be exploited in three Halo 3 Campaign levels:, , 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 Glitch on The Covenant
The Four Arbiters glitch can only be performed on the Halo 3 level The Covenant. This glitch is a combination of two methods (one previously unknown or unexplained) used to attain a second Arbiter. This is also a strategy guide which will help you set up a nice battle between the four Arbiters and up to 10 Brutes (12 on Easy) and 2 Grunts (Heroic and Legendary only). You can see the video guide here.

If you want to get the Arbiters all the way to the end of the level, make sure Cowbell is turned on. Before you go up the elevator to deactivate the third tower's barrier, you will need to have picked up a headshot-capable weapon, a Fuel Rod Gun, and if you're playing on Normal, Heroic, or Legendary, a Beam Rifle (leave it outside). If you started at Mission Start, and the difficulty is Normal, Heroic, or Legendary, pick up the Active Camo; you can find near the first tower's door.

When you have deactivated the third tower's barrier, run past the Flood and activate the elevator. When you come down, you should see Arbiter and any surviving Elites in combat with the Flood. Help them kill the Flood. Now melee Arbiter's shield until his shield goes down. Then go in front of the glass screen and headshot him. Go past the glass screen. You should see a waypoint; a second Arbiter will be spawned with a Energy Sword as his primary weapon. On Heroic or Legendary, headshot and/or melee the Flood combat forms that ambush you at the door and stick the Carrier Forms with spike grenades, since the grenades are abundant in numbers on this level. Or let the newly spawned Arbiter and the Elites kill them. Don't leave the tower after the Flood are dead. Wait for the first Arbiter to get up and exit the tower.

Then head outside; Flood Infection Forms should spawn and you should see a Separatist Phantom. Kill the Infection Forms 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 the Fuel Rod Gun. Four shots to the Phantom's frontal bulbs and/or tails (the damage is linked) will destroy the Phantom on any difficulty. Now headshot the Sergeant. On Heroic or Legendary, stay out of sight for a while. Then go down and you'll get the loading point. Headshot the Marine wielding the Rocket Launcher. If you started at Mission Start, and you brought any Marines with you on the Hornet, headshot them. Killing the Sergeant and the Marine wielding the Rocket Launcher, and any Marines you brought to the third tower will make sure the two Arbiters get in the Warthog. If you're playing on Normal, Heroic, or Legendary, retrieve the Beam Rifle you left outside. Now get in the Scorpion tank. The remaining Marines will board the tank and the two Arbiters will 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. However, the War Chieftain will be unaffected by this so run him over as you go. After you have killed the two Scarabs, the third Arbiter will be spawned. Drive the Warthog to the waypoint. You'll get a checkpoint. Save and quit. Then resume and drive the Warthog to the Citadel entrance. Get the Arbiter operating the Gauss turret off and melee him off the light bridge. Get in the driver's seat to get the third Arbiter to hop in. Then get him off and do the same to him. Do the same with the last remaining Arbiter. Now all three Arbiters should be on the ground level and they should be stationary. Optionally, you can go down to the Arbiters and melee them into the Citadel. Go into the Citadel.

After the cutscene, you should see Arbiter wielding the Plasma Rifle as his primary weapon and the Energy Sword as his secondary weapon; this is the fourth Arbiter. The fourth Arbiter cannot be killed during the fight with the Covenant. Your objective is to reach the third bridge. On Easy, let the Tank Forms do the fighting. On Normal, help them. On Heroic or Legendary, go the side and wait for the Tank Forms to die. After they have died, you have two choices. You can wait for the Covenant to take out Arbiter's shield or just melee Arbiter until his shield goes down. When his shield goes down, he will usually take out his Energy Sword. Thanks to his invincible state, the fourth Arbiter will easily slaughter the enemies on the first bridge. However, you have to stay with him or he'll switch back to the Plasma Rifle. On Heroic or Legendary, two additional Heavy Grunts wielding Fuel Rod Guns will be spawned. While Arbiter fights the Brute Captain, Grunts, and Jackals, go to the door to spawn the two Heavy Grunts. If you want to have the four Arbiters fight the Brutes in full force (12 Brutes on Easy, 10 Brutes on other difficulties), after all of the enemies are dead, kill the surviving Tank/Stalker Forms (melee the Tank Forms in the mouth) and the Human Combat Forms. Then jump on a supply case and jump onto the roof. Now you have the following choices to get past the second bridge battle:

If you're playing on Easy, go down onto the second bridge, bypass the Brutes, and go to the end of the second bridge. Use a Fuel Rod Gun or Brute Shot to propel yourself onto the roof.

If you're playing on Normal, use the Beam Rifle to kill the Brute Bodyguards. Then go down and use the Active Camo when you get near the Brute Captain. Go to the end and use a Fuel Rod Gun or Brute Shot to propel yourself onto the roof.

If you're playing on Heroic or Legendary, use the Beam Rifle to kill the Brute Bodyguards. Then go down and use the Active Camo when you get near the Brute Captain. Two Heavy Grunts wielding Fuel Rod Guns should pop up. If you want the fight to be in full force, don't kill them. Avoid the Brute Chieftain's swing, pick up a Regenerator, and use a Fuel Rod Gun or Brute Shot to propel yourself onto the roof.

When you get on the roof, look away from the Ark's control panel. The three Arbiters should spawn at the unactivated light bridge. Then go down onto the third bridge and go to the unactivated light bridge. The fourth Arbiter should appear.

Now you have several choices. You can either participate in the battle, watch the battle as a spectator, or hide. Find cover if you're playing on Normal, Heroic, or Legendary. Fortunately, there's a convenient hiding spot underneath the light bridge's activation panel. You can crouch and hide under there for the entire battle. You can also go next to the Plasma Rifle wielding Arbiter. He'll usually seek cover. If you turned on Cowbell, the Brute Chieftain will usually knock the Arbiters off the third bridge. To make them materialize on the third bridge, simply look towards the Ark's control panel.

If Cowbell is turned on, after the battle, pick up the Brute Chieftain's Gravity Hammer and knock the Arbiters off the platform. Then activate the light bridge. After the cutscene, look towards the fourth Arbiter, who is now killable and then face the third bridge. You should see three Arbiters get up. Now you'll have four Arbiters helping you kill the Flood.

Note: If you get an unwanted checkpoint, do not save and quit. Instead, press the Xbox guide button and sign out. Now you'll be able to resume from the last checkpoint you saved rather than the checkpoint you just got.

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.