I did:
$ darcs get --partial http://www.cse.unsw.edu.au/~dons/lambdabot 

and got:
Copying patch 552 of 552... done!
Applying patch 1 of 552... darcs: bug in darcs!
Impossible case at PatchRead.lhs:56 compiled 13:44:55 Nov 20 2006
Please report this to bugs@darcs.net,
If possible include the output of 'darcs --exact-version'.
tjc@lolcopter:~/Sources/lambdabot/lambdabot3$ darcs --exact-version
darcs compiled on Nov 20 2006, at 13:48:04
# configured Mon Oct  9 15:12:44 EDT 2006
./configure --no-create --no-recursion

Context:

[TAG 1.0.9rc1
Tommy Pettersson <ptp@lysator.liu.se>**20061008175207] 


The same thing happens if I use --complete instead of --partial.

This is a duplicate of issue364. 

The test case is not currently reproducible with either 1.0.9 or 2.0.0pre2,
possibly because the repository mentioned has changed its contents.