Working on a Story, but then ERROR!

[RedMser]

Could be that something you placed in your map caused it. Happened here too, but it went away. I had a monster, next time I loaded the map in the editor, it said this error, and then I found that the monster I placed went poof. I don't know, but try checking your map, see if something disappeared. If that doesn't help, I don't know what's wrong.

I think I found it... I tried placing a chained bridge, but it dissapeared. I think I did something wrong there. So, thanks for your help!