Darcs asked me to report this, so here it is.  In case it helps, I'm
running Mac OS 10.4.8 and installed and built darcs via macports.

kmac:~ kagrama$ darcs get --partial http://www.iolanguage.com/darcs/Io
Copying patch 666 of 666... done!
Applying patch 666 of 666... done.
darcs: bug in darcs!
Impossible case at PatchRead.lhs:56 compiled 11:35:11 Jan 11 2007
Please report this to bugs@darcs.net,
If possible include the output of 'darcs --exact-version'.
kmac:~ kagrama$ darcs --exact-version
darcs compiled on Jan 11 2007, at 11:37:19
# configured Fri Jun 16 14:55:21 EDT 2006
./configure --no-create --no-recursion

Context:

[TAG 1.0.8
Tommy Pettersson <ptp@lysator.liu.se>**20060616160213]
kmac:~ kagrama$

Hi, thanks for this report.  I'm going to resolve it because it seems to be a
duplicate of issue394 (which I sadly cannot reproduce).

This test case has gone 404, because the developers switched to using 'git', so
it's not helpful for tracking down the bug, either.