Please read this opening post http://forums.kleientertainment.com/topic/69014-orphaned-unnamed-resource/
The logfile is attched to the second post.
It seems this error only accures in the logfile, after shutting down the game? At least that is my assumption why it take so long for me to see it happen again.
Unfortunately I don't have a reliable way to reproduce it =/ It does not seem to be realted to a single mod, but may happen (or may not happen) with alot of mods, when exiting the game. I did not notice any problems, only the error in log_file.