From Halopedia, the Halo wiki
This is a list of minor glitches in the Halo games.
Hole to Hell
The "Hole To Hell" glitch, or A.K.A "Portal To Zanzibar", is just a glitch that makes a large black hole surrounded by a magenta glow which creates a lag-like effect when a player is near it and moving. In order to correctly experience the glitch, ensure that you perform it in an area that is easily accessible.
To perform the glitch go to any map, make sure you have a Gauss Warthog and a Spectre, this could be done with up to two of each, or just one of each. Have one person get in each turret and fire at a wall/ground/mountain/base. Make sure each person is firing at the same spot on the wall. Soon after firing at a spot, it will be just a big black hole with a magenta glow around it and the lag-like effect, but only when in motion near it. It can also be done in Halo: Combat Evolved but it's black with blue surounding it instead. To do the glitch in Halo: CE you should simply use many needlers.
While playing the level Relic on Halo 2, use the Warthog's M68 Gauss Cannon to shoot the sniper platform above the mini-grav-lift (visible at http://halopedian.com/Image:Relic.jpg ). If done correctly, it should fall off and enter the sea, slowly drifting away.
MAC Cannon Glitch
On the level Cairo Station on Halo 2 on any difficulty you can kill yourself in a very interesting way. Fight your way through the level as you would normally until you reach the part where you have to disarm the bomb. As you enter the room from the elevator, on the right is the MAC gun loading mechanism. Jump onto it and allow the MAC round to push you into the chamber, when it fires you will fall through the floor, die, and then fall through the roof of where Master Chief leaves the station in the next cutscene. Sometimes it is possible to fall and appear on the outside of Cairo Station and you fall through space or appear in the right side of the cutscene where you can see it.
Only occurs on Halo 2 and was first documented by Rooster Teeth productions, the makers of the popular machinima cartoon Red vs Blue. Basically, this glitch allows an avatar in a darkened room to appear lit as if in bright sunlight. To achieve this, one has to look at a character, and then if they don't watch them move from a lit area to a dark area, they stay lit (this also works in reverse i.e. a dark character can move to a lit area and stay dark) If the avatar moves forward, or jumps, the effect is instantly lost, although they can be pushed and can move very slowly backwards. Burnie Burns states that this was highly useful in the filming of Red vs Blue at Red Base on the map Coagulation as it is in shadow.
No Arms Glitch
In Halo 3, it is possible to make a player without arms. To perform the glitch, enter forge mode with at least 2 players, one in monitor mode and one in normal. Spawn any weapon like an Assault Rifle. As the player picks up the weapon the monitor has to grab it. It may take a few tries. Then, the monitor player has to move the weapon forwards. The player's arms will grow huge, spin around and dissapear, the monitor then has to delete the weapon (if you don't delete the weapon, it will turn invisible). Strangely, the armless player is invisible if viewed from certain angles.It is also possible to flip an Elephant with the armless player. Get the armless player to stand in front of the Elephant, and slash him with an energy sword. The game should lag and the armless body will float slowly towards the Elephant, and make it violently shake and flip.
In Halo: Combat Evolved, if you kill something in a position where a body will fall half on a ledge and half in midair, the part hanging down in midair will distort and sag down. This is faulty geometry more so than a "glitch".