No weapon glitch

The No Weapon Glitch is a glitch present in all the Halo games. It allows players to apparently disarm their in-game characters.

Halo: Combat Evolved
The No Weapon Glitch is not as well known in Halo: Combat Evolved's multiplayer, but the process is pretty simple. It works easily on Halo PC, but is harder on Xbox because there is more lag. There are two kinds of No Weapon Glitches in Halo PC: one from the host's perspective and one from the client's perspective.

Multiplayer
To perform the glitch in multiplayer, you need at least one other player to work with. Gather multiple different weapons together in a closed area, like in the Blood Gulch Base. Then, have both you and the other player crouch on the pile of weapons facing each other. Consequently, have both you and your partner tap X (the button you have set for the "Drop Weapon" command) repeatedly over the weapons. This should make you continuously drop and pick up new weapons.

Eventually, you will not be able to drop your weapon. This is due to the system trying to realistically process you picking up an object that does not exist. This occurs because you cannot drop something you aren't holding (because it doesn't exist). All other players in the game will view you as if you had no weapon; the player you performed the glitch with should be similarly unarmed.

If you press Tab, you may switch to your other weapon. The game may automatically switch you to your other weapon after a certain amount of time. If you are a client you will find that you cannot switch back to your original weapon -- this is because it no longer exists.

Campaign
The No Weapon glitch is more difficult in Halo: Combat Evolved's Campaign. It involves overloading the map with objects, such as weapons, Grenades, and bodies, so that objects on the map start behaving erratically. Overloading is easiest on the Xbox version in Co-op; simply kill your teammate over and over until their corpses accumulate. With a little luck, you will de-load your weapons. You can also de-load enemies' weapons using this same method.

Observations
If, in Multiplayer, you observe a client who has no weapon, they will assume the animation for having no weapon, like in the first Campaign level, Pillar of Autumn.

However, if you observe a host who has no weapon, they will look very different. The host assumes a phantom weapon stance. They are actually still holding their weapon, but you cannot see it. If they were holding, say, a sniper rifle when they lost their weapon, then their arms will look like they are holding a sniper rifle. They can still shoot and melee with their weapon, and if they switch weapons, you will see a weapon floating in the air where they switched. They can also switch out their invisible weapon for other weapons, changing how they hold their hands in the air.

Multiplayer
In Halo 2, the glitch requires an Oddball and a Plasma Pistol. Find an Oddball, and charge your Plasma Pistol. Walk over the Oddball and pick it up while still charging the Plasma Pistol. If you did it properly, your weapon will have been deleted -- your hands will be empty, and no reticule will be present on your HUD. If you come into contact with any weapon, you will automatically pick it up.

Campaign
This can also be done in campaign. The level Outskirts is highly recommended because the Blind skull can be grabbed on any difficulty, or Regret because the Assassins skull is always there, and it's ability will only work on Legendary. First, do the glitch where you hold three of the same weapon with the plasma pistol. Drop your dual wielded one. Switch between your weapons to be sure that you have two plasma pistols. Then go to the level's skull. Do the trick as mentioned above but with both plasma pistols. You should now be holding no weapons, but with the abundance of weapons in campaign it will be very difficult to avoid weapons on the ground so I would carry the level's skull with you so you can redo the glitch.

Observations
This can be used for some fun game types if everyone cooperates or can be used for making machinima. Please note, however, that unarmed players using the Spartan II player model can still throw grenades while Elites can't; this is because Elites don't have an animation for performing this action when unarmed.

Halo 3
This glitch can be achieved in Halo 3 as well. It mainly occurs during Custom Games, but it occasionally happens in Matchmaking, often during Capture The Flag battles.

Furthermore, it is possible to mod a gametype so that the default weapon is a support weapon, such as a Flamethrower or a Missile Pod. When starting a match with such a gametype, dropping the item before picking up any other weapons can leave the player unarmed.

It can also be done in Forge, but requires at least two players. Have one player go into monitor mode and the other in player mode. Then, find or make a weapon, and get one player to pick it up, and at the same time have the monitor pick it up, and then, get the monitor to delete the weapon. This will make the player pick up a "deleted weapon"

Halo: Reach
This glitch is very simple to perform in Halo: Reach.

Make sure you are holding only one weapon, then pick up a detached Machine Gun Turret or Plasma cannon. Then, while walking backwards, tap the Armor Ability button and the melee button at the same time. You should now be weaponless.

This can work with any armor ability, though it is easiest to do with Evade. Also, using Armor Lock or Drop shield requires you to be jumping backwards instead of just walking backwards, as otherwise you will pick up your weapon again.

Trivia

 * In all the games in the Halo trilogy, John-117 is seen weaponless during at least one level in the campaigns until he gets a weapon, or ammo for a weapon he's been given.