In a local copy -- made with --no-pristine-tree -- of a repository:

21:39:32$ darcs tag --checkpoint 
What is the version name? Appendix B enforced!
darcs: bug in darcs!
fromJust error at DarcsRepo.lhs:522 compiled 20:49:10 Jul  1 2007
Please report this to bugs@darcs.net,
If possible include the output of 'darcs --exact-version'.

21:40:47$ darcs --exact-version
darcs compiled on Jul  1 2007, at 20:50:21
# configured Sat Jun 16 09:59:48 PDT 2007
./configure /usr/local/share/config.site /usr/local/etc/config.site

Context:

[TAG 1.0.9
Tommy Pettersson <ptp@lysator.liu.se>**20070603213706]

Thanks for the report, Jon. 

If you still have this repo around, try testing with Darcs 2 to see if the issue
has been resolved.  You can get binaries for Darcs 2 for Mac and Linux from here:
http://wiki.darcs.net/index.html/DarcsTwo

Once you have it installed, you can create a copy of the repo with the darcs-2
internal format:

darcs covert orig_repo new_repo
cd new repo
darcs tag --checkpoint

Jon, 

This bug is being deferred now, because we need further testing on your end to
proceed. 

If you still have this repo around, try testing with Darcs 2 to see if the issue
has been resolved.  You can get binaries for Darcs 2 for Mac and Linux from here:
http://wiki.darcs.net/index.html/DarcsTwo

Once you have it installed, you can create a copy of the repo with the darcs-2
internal format:

darcs covert orig_repo new_repo
cd new repo
darcs tag --checkpoint

Hi Jon,

This bug appears to have been resolved in darcs 2.0.0 (but you'll have to
convert to the darcs2 format).

I am guessing that this was a partial repository where it happened, and that
darcs 2's --lazy substitute will work better.

I'm going to close this bug, but please protest if you think this may still be
valid!