No weapon glitch
From Halopedia, the Halo wiki
Halo: Combat Evolved
The No Weapon Glitch is not as well known in Halo: Combat Evolved, but the method is pretty simple. It works easily in Halo: Combat Evolved's PC port, but is harder on Xbox because there is less lag. The glitch may be performed in both Multiplayer and Campaign.
There are two kinds of No Weapon Glitches on Halo PC's multiplayer: one from the host's perspective and one from the client's perspective, both of which 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 players crouch on the pile of weapons while facing each other. Have both players swap weapons repeatedly over the stockpile. This should make both players continuously drop and pick up new weapons.
Eventually, a player will not be able to drop their weapon because the weapon starts "phantoming." All other players in the game will view them as if they had no weapon; the player they performed the glitch with should be similarly unarmed. The host may switch to their other weapon, but the game may automatically force a weapon switch after a certain amount of time. Clients will find that they cannot switch back to their original weapon because it has been deleted.
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 have a player kill their teammate over and over until their corpses accumulate. With a little luck, both players will despawn their weapons. Enemies' weapons can also despawn using this same method.
If a client who has no weapon is observed in Multiplayer, they will assume the animation for having no weapon, like in the first Campaign level, Pillar of Autumn.
However, if a client observes a host who has no weapon, they will look very different. The host assumes a phantom weapon stance, as they are actually still holding their weapon, but clients cannot see it. The host's pose inherits the pose of the equipped weapon; 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, clients will see a weapon floating in the air where they switched. They can also switch out their "phantomed" weapon for other weapons, changing how they hold their hands in the air.
In Multiplayer, load up a game of Oddball where players spawn with only a Plasma Pistol. Find the Oddball, and have a player charge their Plasma Pistol. Walk over the Oddball and pick it up while still charging the Plasma Pistol. If done properly, the player charging their Plasma Pistol will drop both the Oddball and weapon: their hands will be empty, and no reticle will be present on their heads-up display. If a player without a weapon comes into contact with any weapon, they will automatically pick it up.
This can also be done in Campaign, but requires a Skull. The level Outskirts is highly recommended because the Blind skull can be grabbed on any difficulty, and is available at the start of the level. First, have the desired player perform the Triple wielding glitch so that they have two Plasma Pistols. Find the Skull, and just like in Multiplayer, pick it up while charging one of the Plasma Pistols. Perform this glitch twice, as the player will still have one weapon. After the second time, the player should not be holding any weapons. However, with the abundance of weapons in campaign, it will be very difficult to avoid weapons on the ground. It is advised to carry the skull through the level to redo the glitch in case a weapon is accidentally picked up.
This can be used for some fun game types if everyone cooperates; it can also be used for making machinima. Notably, unarmed players using the Spartan player model can still throw grenades, but Elites can't; this is because Elites don't have an animation for performing this action when unarmed.
The first method requires at least two players in Forge, either in split-screen or over Xbox LIVE. Have one player go into Edit Mode, while the other remains in Player Mode. The player in Edit Mode should spawn a weapon and place it on the ground. The other player should equip the weapon, and the editing player should delete the weapon at the same time. If performed correctly, the player in Player Mode should have dropped their weapon, and if performed incorrectly the player will pick up the weapon on the ground. If the player has two weapons, the glitch must be performed twice. Note that as in Halo 2, a weaponless player will automatically pick up any weapons they walk onto.
To have no weapons in Custom Games, a modded gametype where players start with a flamethrower is needed. In order to create such a gametype, one should follow this guide on how to mod a gametype in Halo 3. Once in-game, simply drop the flamethrower to have no weapons.
This glitch is quite simple to perform in Halo: Reach, but is only achievable by the host or their guests. The first method requires an armor ability and a detached machine gun turret or Plasma Cannon, while the second method requires the player to be an Elite in Firefight.
In any game mode, have the desired player 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. The player should now be weaponless, unless they had a secondary weapon, in which case they will now be holding only one weapon and need to perform the glitch again. The glitch is compatible with any armor ability, but is easiest to perform using Hologram or Jetpack. To perform the glitch with Armor Lock or Drop Shield, the player must jump backwards instead of just walking backwards; otherwise, the player will pick up their discarded weapon once their armor ability expires.
Alternatively, in Firefight, if the desired player is an Elite and encounters a Elite General wielding a Plasma Cannon, they can give him their weapon in trade for the turret. The player can then drop their turret and have no weapons. If performed correctly, the player will only be able to throw grenades. Also take note that as in Halo 2 and Halo 3, weaponless players will automatically pick up weapons, and will have to repeat the glitch once again to drop them.
Unlike Halo 3, this glitch will lead to a few animation errors:
The glitch may be performed in any game mode, but is easy to do in Forge. In order to perform this glitch, the player must have an armor ability equipped, though a Hologram or Jetpack works the best. Just like in Halo: Reach, the player must pick up a detached Machine Gun Turret or Plasma Cannon. Now, walk backwards, melee, and deploy the armor ability simultaneously. This will cause the player to drop their current weapon. If the player has two weapons, this must be repeated again. Take caution not to walk over any weapons on the ground, as they will automatically be picked up. Also, in gametypes when the player spawns with random weapons, sometimes the player will spawn without a weapon. Players without weapons will still be able to sprint and use armor abilities, but will no longer have a weapon reticle.
As in Reach, this glitch will lead to some animation errors:
Halo 2: Anniversary
There are two ways of having no weapons in Halo 2: Anniversary's Multiplayer, the second of which requires Forge.
For the first method, have the desired player pick up a speed boost to ease the process (it's likely possible without, but much harder). As in Halo: Reach and Halo 4, pick up a detached Machine Gun Turret. Jump backwards, melee, and press the "dual wield" button all at the same time. The player should drop their weapon. If, after performing this glitch, the player is holding only one weapon(or dual-wielding the two weapons) simply perform the glitch again. Like in every other game, walking over a weapon instantly picks it up.
The second method requires a little bit of Forge scripting. In Forge, a few objects must be spawned through Edit Mode first:
Once this is all set up, save the map, then load up the map in Custom Games. Pick up the two weapons scripted earlier, and activate the switch. Once done, enter the vehicle and upon exit, the desired player will have no weapon. A video guide of this method is available here.
Halo 5: Guardians
The No Weapon glitch in Halo 5: Guardians is only achievable in Forge, and requires two players. As Halo 5: Guardians does not feature split-screen, this glitch must be performed over Xbox Live.
To start, load a map in Forge, preferably a mostly empty one such as Alpine, Parallax, or Glacier. This glitch is achieved by overloading the game, and an efficient way of doing which is with Kill Balls, specifically the Kill Ball: Classic (10x10x10; jr). Spawn a Kill Ball, then to make the process easier, duplicate it about 20 times, select those Kill Balls, and duplicate that stack over and over. Keep spawning Kill Balls until the "Objects" meter on the bottom left of the screen reaches around 1200/1600 objects. When finished duplicating, have the desired player stand in view of the Kill Balls, and get assassinated by the second player. If done correctly, the player who is assassinated will not die, but instead drop their weapons. Once the desired player has dropped their weapons, they should be careful not to walk over any weapons as they will automatically be picked up, so they will have to perform the glitch again. Note: this method will only work within the same Forge session that the map is overloaded, so saving the map for later use will only lead to the player dying when getting assassinated. A video guide on this method is available here.