My Emacs mode just broke with a real example of this, but here's a
simple case.  I recorded a trivial file with a blank first line and got
the output below with "unknown" in the label.

  1: Fri Sep 28 18:05:41 BST 2012  Dave Love <d.love@liverpool.ac.uk>
    * x
  
                    unknown |  
  d.love@liverpool.ac.   #1 | a 
                            | b 
  
  
With --machine-readable it's

  19700101000000-0000-0000000000000000000000000000000000000000.gz |  
  20120928170541-0ed77-bad9ce1948ceaad386ea5ed1c9bb000974504be2.gz | a 
  20120928170541-0ed77-bad9ce1948ceaad386ea5ed1c9bb000974504be2.gz | b 
  
  
Incidentally, there's a trailing blank line in both types of output; I
wonder if that's intentional.

Ouch. I think this might be another symptom of
http://bugs.darcs.net/issue2160

The output with machine-readable is a "default patch name", used when
the actual patch is unknown.

The trailing newline is probably not meant to be there.

This has been fixed with the new annotate implementation.