Transparent Scenery Glitch
From Halopedia, the Halo wiki
To trigger the glitch, position yourself (the monitor) along the lower ceiling, where the teams would spawn on Team Slayer, and face straight ahead so that your line of sight is more or less parallel to the lowered ceiling and you are facing in the direction of the team spawn points. Go to the scenery menu and select an item (this works particularly well with stairs, but some items will not work at all). If done correctly, the spawned item should appear in the center of the map closest to the side the forging player is on. The player who forges the item will be unable to see it unless he exits the game and then views it in Theater mode, so this works much more effectively if there is another player working as a spectator. Letting go of the object will remove this effect, and sometimes launch the item a short distance across the map.
The reason for the phenomenon is the fact that the physics engine encoded into the Halo disk can not decide where the item should be placed, so it "jumps" between two spots, and is therefore only there half the time, explaining both why the box sometimes appears in two places and why the object being forged appears transparent.