To witness this rather odd easter egg, play Co-Op with a friend, or on Xbox LIVE. Go to the last level called "Halo". Right after the cutscene, get a rocket-launcher from the downed Pelican. Travel along the path as you would if you had the main goal of beating the game. Before you reach the flood, there should be a series of long pipes, jump over to the right of them on a high-peak(do this as you would if you were finding the terminal on this level). You should see an open entrance, go inside. You should see a large, black pit. Have the person with the rocket-launcher stand behind the other player, the player should run twoards the pit. When the player is running, the player with with the rocket-launcher should fire at or near the running player's feet( make sure you don't fire too close that the running player dies). While the player is flying in mid-air, or somehow lands, they should have a view of a Bungie employee dancing in his boxers.
P.S. This is very hard, so don't get frustrated if it doesn't work the first couple of times.