iTunes: An unknown error has occurred (-39)

I have been using the "consolidate library" function of iTunes to move all of my music from my NAS to my Macbook. Everything went fine for a while, then I started getting an error to the effect of "Error copying files: An unknown error has occurred (-39)". I didn't take a screen capture, and the exact verbage escapes me. Google didn't turn up anything about error -39.

Poking around the XML file, there were no obvious problems, but it appeared that the problem files were grouped around a time period, several years ago. Over time I have switched between AFP and SMB (Samba) as my protocol of choice to access my music. I recalled running into issues between the two, with regard to special character handling in filenames. I also recalled that the time period in question may have been when I was using SMB.

On a hunch, I reconnected to the NAS, using SMB instead of AFP. I re-ran the organize/consolidate function, and the remainder of my library is currently consolidating without problem.