1. You are using a modded client
If you're using a modded client, unfortunately we cannot provide support for it.
If you have any issues with it, you should report them to the author, as we have no control over the development of it.
2. You have used external game editing tools
If you have used external map editing tools, there's a chance that one of them may have corrupted something in the game.
If the issue you're getting is only pertaining to one specific world, there's a good probability it's "corrupt".
Currently, we have no absolute "fix" for this, if you believe your world is corrupt, you may want to attempt to "undo" those changes with the editor.
3. You are playing on a map that once used mods
If you're playing on a map that once had mods, there's a chance that one of them may have left back map data that can only be 'deciphered' by that exclusive mod.
4. You are playing on a modded server
If you are playing on a modded server, you may want to refer to this page.