This is an fyi (no response necessary).
I got this error and researched it and found no good answer for how to deal with it:
[font="Courier New"]"File ID 5 of database ID 8 cannot be shrunk as it is either being shrunk by another process or is empty"[/font]
One post suggested that it was a memory issue, but it also suggested the only way to deal with it was to restart SQL. I don't have that luxury, so I cleared my cache and reran the shrink and the shrink worked.