Usable Pelican turret
From Halopedia, the Halo wiki
The usable Pelican turret is an Easter egg on the Halo: Reach campaign level Long Night of Solace.[1]
Walkthrough
After the Pelican carrying the Slipspace bomb enters the Ardent Prayer, the player can enter the cockpit and control the turret. To do so, they must crouch low along the side and keep pressing the button to enter, until they do so.
While inside the turret, the player has complete control of it and can fire it in any direction. The targeting reticule matches that of a Falcon. Unlike most vehicles, however, the view is still first-person, and as such the player's vision will be heavily obscured by the turret itself. After taking enough enemy fire, though, the turret model will disappear, and the player will be able to see out and continue firing it, minus the temporary blocking by the firing flash.
If the player is not inside before the turret disappears, they may be unable to take control of it, but once inside they can exit or enter as they like. There is no way for enemies to kill the player when they are firing the turret, and it has been noted as a good way to finish off battles in the hangar on higher difficulties.
This trick can be used again on New Alexandria with the Flyable Pelican and Phantom Easter Egg.
Related Links
Internal
External
Tyrant's Halo:Reach Mythic Guide, Halo.Bungie.org
Sources