I was playing on a LAN world that my girlfriend setup for about 4 hours yesterday: totally reforming the land we previously decided to build a house on, farming, mining, catching animals, the whole 9 yards. We had all this work done and when we go on today, all of that work disappeared. The entirety of the environment around our house was just like it was before we played last night. We both still had essentially everything in our inventory that we had when we stopped for the night, but the world changes were completely undone.