Secret Room On The Ark
From Halopedia, the Halo wiki
The room can only be accessed while watching a saved Film in Theater. Right after the Pelican at the beginning lands, and when John-117 begins to jump out of the Pelican, the viewer may be able to see a glimpse of the room. To get inside, one needs to immediately pause the Film, and fly up as high as possible against the mountain on the right side of the Pelican. Once against it, quickly tapping Y twice will send the viewer straight into the room. The room is actually the bridge of the Assault Carrier Shadow of Intent, commanded by Rtas 'Vadum. The room is seen in three cutscenes: The one between Floodgate and The Ark, the one at the beginning of The Ark, and the one at the end of the game.
This room is not actually an Easter egg; the room itself isn't a glitch either, though the method of accessing it is a glitch. Despite many wild theories on the subject, the room does serve an actual purpose.
In Halo 3, cutscenes are not pre-rendered, but are instead rendered using in-game resources. This means that the cutscenes are not made and added in, but take place in the level itself. Therefore, everything the player sees in a cutscene has to be somewhere in the level, though not necessarily where it can be seen or accessed. This is why if one throws a grenade right before a cutscene, the burn mark will be visible (depending on what you used and where you threw it) and why dead bodies in cutscenes often vary. Cutscene rooms are often placed below the level and large scenes like space battles sometimes happen in a completely different and separate part of the level. The placement of the bridge and the glitch used to get into this room are pure coincidence, and if the bridge were placed in a different place the glitch would not have the same effect. It is strange, however, that the bridge was not placed in the geometry of the first cutscene with the space battle, although the camera effects and movements would probably have easily revealed it.