I was building a giant building wen I suddendly saw when loading the world that a quarter of that building was gone. When I tried rebuilding it and restarted the world, the part I rebuilt was gone too.

After some times on that world there were large worldholes, I couldn't build there and when i flew over or through them, I started lagging. There were more and more such worldholes appearing after the time and none of them dissapeared when I restarted the world.