I was looking for the new stone blocks in the 1.8 snapshot when I saw a square hole in the ground in a desert. It turned out to be a dungeon. But the items were spilled around the room. I dug out all of the sand, and it turned out the chests and mob spawner itself were actually cobblestone blocks instead! It was in singleplayer. This is the only instance i have seen it in, and it does not happen in a desert temple i also found.