Miniature Spartan glitch
From Halopedia, the Halo wiki
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The Miniature Spartan Glitch is a glitch that can be performed in the Halo 4 campaign level Composer. It causes The Master Chief to shrink to a small size.[1]
Method
This glitch is performed in the composer room when you fight against the Covenant. On a platform in the middle of the room to the left is a Banshee. Quickly run to this, and kill the Elite who runs to man it. Fly it over to the building near the hanger opening, lift 03, that you normally would exit when you are done. Park it near the door, and clear out the enemies.
When the first wave of banshees comes, make your way back to your own banshee. After having cleared the banshee, get into your own and wait by the door. Another wave of banshees will follow, but do not destroy any of these. In a short while dialogue between Master Chief and Cortana will be spoken, and, if you are waiting near the door, a loadzone will be triggered. After this occures, fly out to intercept the banshees as they are leaving the map. Just as they are about to leave, board one of them.
As you board it, the banshee will begin to shrink. If you timed it properly, you will shrink along with it. You will remain inside the map, and will be able to pilot the shrunken banshee.
When you dismount, you will find that you have shrunken. As a result, you will find that your point of view has changed to be lower to the ground. Your weapons, however, will remain thier normal size. You can still operate vehicles and turrets.
Trivia
This glitch is similar to a glitch in Spartan Ops that causes enemies to shrink.
Source