Forward Unto Dawn glitch
From Halopedia, the Halo wiki
Forward Unto Dawn Glitch is a Halo 3 glitch. By exploiting this glitch you can get a better look at the Forward Unto Dawn in Theater mode.
Walkthrough
In the Level The Ark when the Forward Unto Dawn arrives and starts unloading its Warthogs and Scorpions, you can get a closer look at it by going into Theater mode. 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. You can also perform a similar glitch on the multiplayer map Sandtrap with the frigate Aegis Fate by overloading the map or using the pan-cam glitch.