This occurs during the manipulation with the list of mp3 files in another window, I think.
Does it happen when you move one of the files which would be visible in the Playlist panel if the panel were not hidden?
I can not specifically repeat the mistake, it is better to view the source code.
Only Ian can do that, so it'd be helpful if you can explain how to make the problem appear. It's not your fault that the problem seems unpredictable, though
I did not think that the problem may be in plugins.