Forward Unto Dawn glitch
From Halopedia, the Halo wiki
Go into floating camera mode and position the camera under the Forward Unto Dawn's cargo bay with your back to the end of the plateau. Go backwards until you are stopped by an invisible barrier then press LB to go down. Keep going until the barrier ends. Go backwards as far as you can and you will be outside the barrier. Now you are free to take a closer look at the Forward Unto Dawn.
This is also achievable through theater mode on the level, Halo, at the very end.