If I have a episode in progress it is locked by the program downloading it thus making it impossible to rename. However because MC sees the avi, it tries to do just that. Unfortunately whilst it fails to rename the avi, it does however rename the nfo.
In this case it would be better to see if the avi can be renamed & if so then rename the nfo.
PS Please don't remove the auto-rename function as it is otherwise brilliant!
PPS Just one more thing, when you manually rename the screen doesn't update so you don't get any feedback as to whether it has worked or not. Perhaps a popup message to indicate that the episode can't be renamed. If so it would probably have to handle if you have multiple episodes that can't be renamed when trying to rename whole seasons.
If the file can be renamed, then the screen just needs to be refreshed so that the renamed nfo can be seen.