I was about to send-pr this, but I'm not quite sure it's really wrong.

If you mv a file between filesystems and don't own the original file,
mv prints a complaint about not being able to set the ownership back the
way it was.  (Aside from the message, it "successfully" completes the
operation, including deleting the original (if it can).)

The mv manual page defers to cp(1) for the cross-filesystem case (even
though the actual mv code no longer defers to the /bin/cp binary...
;-).  The cp(1) manual page indicates that no message will be printed
if you simply cannot set the ownership; the implication, to me anyway,
is that mv should not do so, either.  (This will certainly reduce the
number of gratuitous messages appearing in /var/mail/news from scripts
doing mv operations...)

I have a patch for mv (ripped right out of cp) to make it agree with
the above interpretation.  Unless there's some genuine reason to keep
the message, I'll send-pr this.