I had a save running on minecraft 1.7.9 with optifine, and in one point I moved to play on the latest snapshot, 14w21b. When I opened the save again with minecraft 1.7.9 my whole inventory was gone, all the cobblestone entity that was thrown on the ground became normal stone and all my chests, hoppers and furnaces became empty. I hope you fix it soon, it is hard right now to move between the snapshots and 1.7.9! Thanks!