When I was playing in my singleplayer world I died many times besides a zombie spawner. But when I closed Minecraft and opened MC again and got back to the place I died all my items on the ground were turned into smooth stone.

EDIT: Just realized that I made the world in the snapshots (14w03a) and opened it in 1.7.4 might be because of that altough I still don't understand why this happened. Also noticed that stuff from chest and furnace were gone.