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

Three Arbiters Glitch on The Covenant
The Three Arbiters Glitch or The Early Two Arbiters Glitch can only be performed on the Halo 3 level The Covenant. When the glitch is performed, the player can get two Arbiters earlier in the level. To completely perform the glitch, pick up the Power Drain.

When you have deactivated the third tower, ignore the Flood and activate the elevator and go downstairs. You will see the Arbiter and the Elites fighting the Flood. Kill the Arbiter and go to the entrance of the third tower, but do not go outside. The second Arbiter will be spawned. When the first Arbiter rises up and leaves the tower, go outside and you will see a Separatist Phantom. Destroy the Phantom. Get the two Arbiters in the Warthog; on Heroic and Legendary, you should give the Rocket Launcher to the Arbiter riding shotgun. Drive the Warthog to the Hornets. It is possible to drive to the Hornets without killing all of the enemies; any enemy that the Arbiters didn't kill will be instantly killed. After you have killed the two Scarabs, the third Arbiter will be spawned. Get the two Arbiters in the Warthog into the Citadel; drive the Warthog to the Citadel entrance and stick the Warthog with a sticky grenade.

After the Cortana Moment, if the player wants to retain four Arbiters instead of three, kill the third Arbiter as you get the loading point.

After the cutscene depicting Miranda Keyes death, get to the third bridge. There will be three or four Arbiters, depending on the player's choice. To bring the Arbiters to the end of the level, deploy the Power Drain and headshot the three Arbiters that can die and activate the bridge. After the cutscene, the Arbiters will remain and help you kill 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.

The Three 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.

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.