I was going to try to come up with something like:

darcs get --partial --tag=2006-10-05 \
    http://darcs.haskell.org/ghc ghc.breakitnow
# darcs says: darcs: Couldn't read tag
cd ghc.breakitnow
darcs pull --tag=2006-10-22 --all
darcs unpull --last=1
# grab foo.dpatch from this issue or whatever
darcs apply foo.dpatch
darcs pull --tag=2006-10-22 --all
darcs optimize --reorder

to reproduce a bug I ran into in my GHC working dir, but unfortunately the very
first command failed. (As the comment afterwords might hint.)

To repeat:

darcs get --partial --tag=2006-10-05 \
    http://darcs.haskell.org/ghc ghc.breakitnow

grabs the wrong checkpoint. (It grabbed the one for the 2006-10-22 tag when I
last tried it. I'm not sure which it should have grabbed, but that one clearly
ain't it.)