I was working on a custom map in 15w33a, and then I saved and exited the map. When I returned, the new snapshot was released, my Minecraft automatically updated, and when I loaded my world again, the entire spawn chunk was gone. At first, I thought it was just a chunk error, so I reloaded the world a few times, and then double checked it in MCEdit. There are no blocks in the area.

I did back it up in the previous snapshot, but it's still hours of work gone.