darcs reported bug by itself:
-------------------------
~/dev/hs-libs/gtk2hs$ darcs opt --checkpoint
Checkpointing tag:
Fri Feb  2 19:38:36 CET 2007  Duncan Coutts <duncan@coutts.uklinux.net>
  tagged 0.9.10.6
darcs: bug in darcs!
fromJust error at DarcsRepo.lhs:533 compiled 21:53:22 Jun 10 2007
Please report this to bugs@darcs.net,
If possible include the output of 'darcs --exact-version'.
^wk@vault108:~/dev/hs-libs/gtk2hs$ darcs --exact-version
darcs compiled on Jun 10 2007, at 22:03:21
# configured Sun Jun  3 18:53:47 EDT 2007
./configure

Context:

[TAG 1.0.9
Tommy Pettersson <ptp@lysator.liu.se>**20070603213706]
-----------------------
It occured as shown, the local repository was a locally unchanged copy of:
http://darcs.haskell.org/gtk2hs
tagged at 0.9.11 and not pulled since that tag in upstream repos.
The command optimize --checkpoint automatically did checkpoint at older
0.9.10.6 tag, not the last one (btw is that expected?)
After the occurence of this bug I issued 'darcs repair' and it reported it
was already consistent.

Attachments

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Thanks for the report. Just checking: this was a --partial repository, right? 
If so, this is a known bug.

Oh, and congratulations! You are our 500th bug on the new tracker.  Sorry, sorry....

I just submitted a regression test update to "partial.sh" which demonstrates
that this bug is "resolved in unstable" using Darcs 2 and a darcs-2 format
repository.