Hall of Mirrors

This glitch can be done on the second part of The Covenant and on Sandtrap, and Isolation.

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.

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.