Hall of Mirrors

The Mirror, Mirror on the Wall glitch is a Halo 3 glitch which can be done on the second part of The Covenant and on Sandtrap, The Storm, Isolation and Halo.

Sandtrap
For the Sandtrap part, this must be done in Forge mode. Go into Forge mode, go into that chasm type thing and face the high dunes that cover up the barrier. Look right and move forward into the indention of top of the wall just below the ledge and zoom in. You will either see a reflection of yourself or what your opponent(s) are doing.

The Covenant
For the Covenant, skip to Rally Point Alpha. Get into a Hornet and fly to the second tower. Look directly at it and hover down to the left. You will be facing a ledge, go there. Park the Hornet onto the longest ledge and face the rock wall. Turn left and move forward to a crack in the rock. When you look in it, you will see yourself or whatever you're co-op partner is doing.

The Storm
For the storm, skip to Rally Point Alpha. Note: must have a co-op player with you. Now get two players on a mongoose and have them drive against a door and the passenger to alight. If done correctly, the game will freeze for 10 seconds. It must be done during the Scarab part. The Scarab will disappear and if you go to the door, you will see a mirror reflection.

Isolation On Multiplayer With A Friend
On a variant of the multiplayer map Isolation, called "A Base". The glitch is on the outer wall; if you venture outside the base and past the Mongoose, look at your feet while sticking to the wall. So far along, your feet will start to float, like you are falling. Start to crouch and you will start to fall. Carry on crouching until you hit the grass. If you turn around so you are parallel to the wall, you will see your Co-Op buddy. This must be done on local with a friend or you will see double vision.

Halo
Skip to Rally Point Alpha so that you are right after the part where you kill 343 Guilty Spark. One player move forwards and kill the flood ahead, and one do not move from where you spawn. The second player should look behind them. You will see two doors. Go to the bottom of the one on the left and look straight up. There will be a small crack that displays a part of the other players screen. This can be done on any difficulty on co-op.