Roaming spectator

The roaming spectator glitch is a trick used to force the Theater camera outside of a map. The glitch can be performed in the Campaigns of Halo 3 and Halo 3: ODST.

Walkthrough
Simply position your character near the top of an invisible wall, and end the game. Watch the newly-created film in Theater, and pause when your character reaches that position. Switch to a third-person view, and rotate the camera to the other side of the barrier. Press Y to detach the camera.

Trivia

 * Using the Brute Jumping Glitch could help in some areas as to getting near the top of the barrier.