Two Arbiters glitch

The Two Arbiters is a glitch that can be exploited on two Halo 3 levels, Halo and The Covenant on single player mode. The result of the execution of the glitch will be that you will find yourself accompanied by two Arbiters.

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 scripted and after the second comes down from the Phantom it wont have a marker above his head. The second one will then be teleported when you get to certain parts in the Citadel.

Another way to do it is to kill the Arbiter just as Truth starts speaking, just before the cutscene when you get into the Citadel after defeating the two Scarabs.

Halo
The first step requires you to be able to kill the Arbiter right before you enter the room where the battle with 343 Guilty Spark is fought. In order to achieve this, you will have to do the Spartan Laser Glitch, and you should preferably play on Easy.

Once your Spartan Laser has infinite ammo, go to the door right before the fight with 343 Guilty Spark, but don't enter. Shoot the Arbiter with the Spartan Laser until he dies. Quickly after that, go through the door and initiate the cutscene. Once you finish the fight with 343 Guilty Spark, go outside and there will be 2 Arbiters. One standing looking at you and another already attacking the awaiting Flood. That one has the waypoint above his head. Once near the end of the hallway both Arbiters will stay back.

On The Covenant
The glitch works on The Covenant because when the Phantom begins to engage its active camouflage, all of the Elites on board unspawn, as can be seen by players who actually get on the Phantom with the Elites. But because the Phantom is destroyed, the camouflage never triggers, and neither does the unspawning. Therefore, the Arbiter remains with you. He spawns again the next time the Phantom shows up.

On Halo
This happens because when you enter the fight it originally unloads the Arbiters model then when finished with the fight it reloads him back to the hallway. But since you kill him the game registers him as not there so therefore not unloading him. And when you finish the fight he is still there and the game still loads the second Arbiter because the game's code doesn't check if the first Arbiter is unloaded.