This error was very common when World Adventures was released because the save file management system was still new and weak at the time (the error itself mainly occurred because the system was quick to get confused and error on the user instead of seeking ways to fix the problem).
Actually, people have been having a bit trouble with this somewhat lately. The thread I saw wasn't that old. World Adventures (the patch, anyways) changed the way save files were created, and that's what made it possible. It seems that something with Showtime may have brought it back up.
