I built on a land months ago and when I came back, parts of it were claimed, blocks were placed, and I spent a lot of time on my second house. The back of my first house which I share with RaZorWoLves is also partially claimed, even if I believe I claimed it! I'm slightly annoyed when I can't removed my blocks and move my items. I spent a lot on materials, and now I come back on to find this.
If you didn't claim the area beforehand, Then it's your fault for this happening to you. If you did claim it, then that could be a problem.. Which world is this in?
