It's in the post -- I added the URL for an MP3 file to the player so it would stream down the podcast, i.e. I did not download a full file and then add it to the library.

I used Clip -> Delete to try to remove it, but it wouldn't delete. It wouldn't play, either.

But anyway, it's no longer a problem since I flashed the new firmware with the SDHC fix on the N800 which erased the old library file.

I still want to know which that file that was in my user directory, so I can edit it if the problem comes up again.