Is it your terrain map or someone elses because.
If for example: you had my Survive terrain, and then I had made changes to it without renaming it, and then I hosted a game and you joined with the different version ( in this case the older version ) you'll have problems.
Thats why terrain names need to be changed if you update them.
Thats why you may see Badlnd1, Badlnd1a, Badlnd1b all are basically the same terrain but with minor changes.
What you could do if you join someone hosting a game using this problem terrain is to quit the host, delete the terrain and then join the host again so that you'll end up downloading the host version.
Pokie