From Halopedia, the Halo wiki
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.
- Using the Brute Jumping Glitch could help in some areas as to getting near the top of the barrier.