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.
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Entirely possible as new entities were added in the snapshot. Older versions not having this information may replace unknown entities with other objects.
Given that the world was created in a snapshot, I can't image that going backwards in versions would be really supported. I can say on the items disappearing that I did notice this in an older world that had been opened in the snapshot, had slime blocks put in a chest, and then opened back in 1.7.4. No issue with things turning into smoothstone that I noticed, but the chest of slime blocks was empty. Much re-gathering of slimeballs to do now...