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I was playing on legendary on high charity and a skull appeared on the last lift of the game right after you pass the huge Brute vs flood battle.here's a pic:http://img294.imageshack.us/my.php?image=highcharityhv5.jpg
Yeah, it's an "Easter egg." You can go here for more information on Halo 2 skulls: http://halo.wikia.com/wiki/Halo_2_Skulls