Yesterday I got the upgrade to 1.75. I played a world a had been in for a while. I went to sleep in the real world and when I woke up the next morning and fired up Minecraft, the chunk I had been in, just that little area, was completely returned to it's initial state. The rest of the world was untouched. I had been in what I call an Enderman Blind out in the middle of a patch of plains: it's a cobblestone tower about 15 blocks high with a lookout platform on top and a ladder through a shileded tube to get up, with a rectangle of blocks on the ground arranged so that when I spot an Enderman at night from the top I can look at him, slide down the ladder, and then when he comes to get me he can only stand in this trench where I can hack his feet but he can't touch me. For something to do during the day, there was a ladder going down deeper, connecting with a tunnel system to the rest of my world.

When I woke up and saw my tower missing, I went to another entrance to my tunnels system and tried to see if I could get it to load if I came from another direction. At a certain point when I was getting close, there was just a wall of stone. I went around and looked at everything else, and it was all there, just like it was. There were ponds all over that field that I had paved using lavabuckets to petrify them, and they were still stone covered. Only the tower was gone.

Wierd. I play on Hardcore, so I back up occasionally by using windows to copy the save somewhere outside of the minecraft folder, but that tower isn't worth going back and losing a bunch of experience, not to mention Ender Eyes and Diamonds, but it's definitely very peculiar. Nothing like it has ever happened.