On the level The Covenant, where you have to get in the hornets (this works best when you destroy the scarabs) go to the edge of the maps where The Arbiter kills the prophet.

You can't go head on, so go down until you can move forward. You are out of the map.

Now go forward 'till you're at the very end and cant go any further (note: sometimes you die if you go too far).

Then go up until you're on top of the building. Go back where you came from, but slightly going up.

Last, you go towards the mountain and keep going until you're screen freezes.

If you a certain USB junk drive, there's a skull there that gives you recon.