Monitor glitch

The Monitor glitch is a glitch in Halo 3, Halo: Reach, and Halo 4. The glitch allows a player to appear as a monitor, and be able to pick up and fire weapons.

Halo 3
To perform this glitch, you must have at least two players on two different consoles start a game of Forge on Xbox Live with unlimited ammo, and primary weapon Spartan Lasers. Find out who is host. The host cannot be the party leader. The person who will become glitched must be the party leader, but NOT connection host. The party leader should then go into monitor mode and then go above the connection host. The host should look up and aim at the monitor with their Spartan Laser, and the party leader should select the host's gamertag on the scoreboard and select the boot player option and continue to the screen where the yes/no option appears. Once the boot player screen is up, have the host fire the Spartan Laser at the monitor. For the next part, the timing has to be perfect, or else it won't work. The second the laser fires, kick the host. You'll know if it worked if you hear the monitor explode, and then the sound of a Spartan's shields dropping. If you kicked the host too early, you will still be alive. If you kick the host too late, you will die. You will know if it worked if you have the shields of a monitor, yet can see the score, instead of the forge budget, and on your radar, you have the blip of a monitor. You will also have no crosshairs. At this point, you are neither a human or monitor. You can press up to go into actual monitor mode and spawn things, and then go back to the glitched mode. The connection host cannot see you in the glitched mode, and if you have a weapon, all the host sees is a floating weapon. The only things that reset this glitch are getting killed and possibly the host leaving. You have the movement controls of a monitor, but you can fire weapons.

Halo: Reach
The glitch can also be performed in Halo: Reach.

To replicate this glitch, start a Forge game with at least two people over Xbox LIVE. Go into editor mode and you should notice that for a split second you have a weapon crosshair inside the Forge crosshair. You should also see a Focus Rifle appear in the top right corner for a split second. This glitch will allow you to fire that Focus Rifle, although only for a split second. As soon as you go into editor mode, press the right trigger. You should hear and see a Focus Rifle shoot for a second. This shot can and will damage players if aimed at them, and, if it kills a player, the message, "Killed by the Guardians" will appear.

Halo 4
In order to perform this glitch in Halo 4, the player must switch from editor to player mode and fall from a great height. Just as the player hits the ground and is about to die, they must switch back into editor mode. When they try to switch back to player mode any time after this, all that will happen is that the monitor's reticule will disappear, editing options will be unavailable, and the player will be able to pick up and fire weapons. While technically in player mode, the player will still retain the characteristics of a monitor, viz., they have the movements of a monitor, they shall appear as a monitor to other players (merely with the addition of a weapon), and their HUD shall appear to be that of a monitor. The only difference being that the arms of the Spartan, holding the weapon, will be visible in the same way they would normally in player mode.

Trivia

 * Performing this glitch with a Sentinel Beam or a Spartan Laser will make the monitor look like 343 Guilty Spark, who fires his laser throughout campaign.
 * When the player attempts to perform this glitch on the Gravity Hammer, switching weapons and switching back will make other players see a mini-hammer going through the monitor.
 * Sometimes, if you fire your weapon, players may see projectiles coming from random places on the map.
 * You cannot use this glitch on mounted turrets, but if they are dismounted, you can pick them up and shoot them.

Source
