trentbuck,

Thanks for the report. With your helpful test sequence, I was able to reproduce
the bug and have confirmed it still exists with the Darcs 2 pre-release. 

It seems like a simple solution would be disallow unrecording checkpoint tags. 

  Patient: Doctor, it hurts when I do this.
  Doctor: Then don't do that!

I'm attaching a test script that a developer can add as a regression test when
they are ready to work on this bug.