I logged into my single player world in 14w03b and I had my full inventory and my chests had the correct things in them. I logged out and went back to 1.7.4 to play some PVP. I came back in 1.7.4 into my single player world. My inventory was wiped of all my tools, as well as my armor. I went to check some chests and they were empty. Knowing that I had stuff in them. My ender chest was also whipped of all its belongings. Including diamonds, swords, rails, minecarts, pickaxes, and armor. Not all of my chests were whipped but most of them were. Some chests had wood in there, wool, pumpkins, wheat, bonemeal, seeds, leather, etc.. My tool chest had all of my tools in there. So all the things in my inventory, including boots I was wearing got whipped, and it seems all the chests in my surroundings where I spawned in, probably in a 20x20 around where I spawned in, that includes my ender chest, which was out of that range.

I thought I would have all my items that I logged out with, as well as all my items in my ender chest and other chests.

My ender chest was whipped of all its items, they all disappeared as well as all the items in my inventory and some of my normal chests got whipped too.

Steps to Reproduce:
1. log into a single player world in 14w03b and make sure you have items in your inventory, as well as some diamond armor pieces. Also make sure you have some items in your ender chest.
2. go back into the same world but this time in 1.7.4.
3. If it recreates what happened, your inventory would be whipped as well as some chests and the ender chest.