Weaponless Elite glitch
From Halopedia, the Halo wiki
The weaponless Elite glitch is a glitch which can be done on the Halo: Combat Evolved level Assault on the Control Room, in Halo 2 levels Metropolis and Gravemind, Halo 3 level The Covenant and any campaign level in Halo: Reach that includes an area where an Elite and driverless vehicle meet.
Halo: Combat Evolved
Assault on the Control Room
Go through the level until you get out of the caverns, walk up the big slope and reach another canyon with a giant pillar in the middle. Walk inside it and you should see a Stealth Sangheili. When it notices you, run out and it will follow you. Sometimes the Elite will run straight outside if there is a ghost there (if the player brings one from the first part of the level, the one parked outside by default will not attract it). The Elite will get into the ghost and will try to shoot and ram the player. If the player manages to tip over the ghost, the Elite will fall out and will have no weapon and will not even move or react to the player, unless they gets close, where the Elite will try to melee with his bare hands as if it had an Energy Sword.
Go to the level Metropolis in Halo 2 and board the Scarab. Throw a Anskum-pattern plasma grenade at the Scarab operator Elite while he is operating the Scarab. He will execute the animation for drawing an Energy Sword but will not actually possess one. Easy is the best difficulty for this.
Get the Catch and Sputnik skulls and go to the level Gravemind. Make sure that the difficulty is on Easy. Go to the first outdoor area past the Hunters where the Honor Guard Elites and the Brutes are fighting. You should take out the sniper Jackals before attempting to perform this glitch. Get the SpecOps Elite toward the right to throw a plasma grenade at you. Either shoot a Brute Shot at the Elite's feet or jump above his head, but just enough so he does not stick you, and cause him to throw the plasma grenade up in the air. If done correctly, the plasma grenade will land on his head and not you, and he will execute the animation for pulling out an Energy Sword, and again will not actually possess one as this Elite is Special Operations and is not scripted to have a secondary weapon. Instead, he will pull out an invisible Energy Sword and will follow you around the Valley of Tears. When you reach the part where the Flood-controlled In Amber Clad comes, he will not follow you across the light bridge but stand there staring at you.
When the Separatist Phantom arrives to pick up the Arbiter, use an Energy Sword to melee and destroy the Pek-pattern plasma cannon platform. The Elite operator will not take any damage and when he gets off the turret, he will have no weapon. If you stick him with a plasma grenade, he will take out an Energy Sword and usually die. If the Thunderstorm and Mythic skulls are turned on and you stick him with a plasma grenade, it is very likely that he will survive. An easier way to have the weaponless Elite is to destroy the Phantom by rapidly firing your Fuel Rod Gun in the engines, however, the weaponless Elite has depleted shields, and the Arbiter will die. If you betray your allies, he will attempt to melee you with his fist and his melee does damage.
The best example of this glitch is on the level Winter Contingency. First, acquire a Plasma Pistol before entering the Falcon that would take you to Visegrad Relay. Once the Falcon lands on the Relay's courtyard, charge the Plasma Pistol and get ready to disable the Falcon with an EMP blast. When the Falcon begins to fly away, fire at it with a charged Plasma Pistol shot. This shot needs to be precise, so that the Falcon will land in front of the Relay's gate, and get stuck under an extension over the gate. The Falcon will attempt to fly up, but with no prevail.
Now clear up the courtyard and wait for more Covenant. Once the Spirit lands, it will drop a lance of Covenant led by an Elite. Lure the Elite to the Falcon, either by approaching it and back off as it melees you (recommended so you will not die from the Covenant fire) or take down its shields and make it go berserk (not recommended because Noble Team members may kill it). Once near the Falcon, divert the Elite's attention from you to the Falcon either by hiding or running away (the latter may not work). The Elite might fire at the Falcon for a while, but it will eventually board the Falcon. The Elite will open the cockpit, but no pilot will be inside. It will leave the Falcon unmanned and be weaponless (An Assault Rifle can be found floating beside the cockpit's right side for unknown reasons). Sometimes it may jump back on the Falcon and start to pilot it, slightly hovering above the ground. It may even fire the Autocannon towards it's enemies, including the player.
If the Elite doesn't jump back on the Falcon, it will walk up to its enemy, remain hostile but will not attack. If played on the solo campaign, Jun-A266 will be riding the Falcon and will leave the Falcon passenger seat after the Falcon is boarded and enter the cockpit of the Falcon, continuing the flying sequence.
This glitch can be also performed on Tip of the Spear, when Jorge-052 arrives at the second Tyrant to pick up the player. Though it rarely happens, Jorge's Falcon will land before the area is clear, and the Falcon will be boarded by an Elite, which will result in the same weaponless Elite.
Because the Elite's grenades are weaker than yours, it will not kill him but will make him execute the animation for drawing an Energy Sword, causing him to become "weaponless".