Running
 	darcs get http://code.haskell.org/yi
produces one of the following responses (depending on version).

Running the darcs-1.0.9-i386-linux binary, as downloaded from darcs.net, 
produces the message:

$ darcs get http://code.haskell.org/yi
...
FINISHED --18:23:31--
Downloaded: 5,562,103 bytes in 1370 files
Applying patch 551 of 1370... darcs: bug in darcs!
Impossible case at PatchRead.lhs:58 compiled 18:47:46 Aug 28 2007
Please report this to bugs@darcs.net,
If possible include the output of 'darcs --exact-version'.
$ darcs --exact-version
darcs compiled on Aug 28 2007, at 18:50:44
# 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]
$

Alternatively, after downloading and building darcs from the repository 
using
 	darcs get --verbose --partial http://darcs.net/repos/stable
the error message has moved slightly:

$ darcs get http://code.haskell.org/yi/
Directory '/home/mga83/working/Haskell/yi' already exists, creating 
repository as '/home/mga83/working/Haskell/yi_0'
Copying patch 1370 of 1370... done.
Applying patch 551 of 1370... darcs: bug in darcs!
Impossible case at src/Darcs/Patch/Read.lhs:58 compiled 18:37:35 Jan 15 
2008
Please report this to bugs@darcs.net,
If possible include the output of 'darcs --exact-version'.
$ darcs --version
1.1.0pre1 (unknown)
$ darcs --exact-version | wc -l
1433
$

By the way, the page http://www.darcs.net/manual/node3.html suggests 
downloading the darcs repository from http://abridgegame.org/repos/darcs 
-- this appears to be a case of bit rot!

I hope this report is helpful.  Unfortunately searching bugs.darcs.net 
doesn't seem to work and just hangs.

On Wed, 16 Jan 2008, Michael Abbott wrote:
> New submission from Michael Abbott <michael@araneidae.co.uk>:
>
> Running
> 	darcs get http://code.haskell.org/yi
> produces one of the following responses (depending on version).
>
> ...
> Impossible case at PatchRead.lhs:58 compiled 18:47:46 Aug 28 2007
> Please report this to bugs@darcs.net,

I'm not at all sure what the proper process for handling these bugs is (I 
see that this has created two issues, 589 and 590).  I posted a follow-up 
to darcs-devel, so hopefully my reply here is redundant (in which case, 
apologies for the extra noise): the underlying trigger of the message is 
our web proxy server quietly transforming the offending patch into 
garbage!

On Wed, 16 Jan 2008, Michael Abbott wrote:
> New submission from Michael Abbott <michael@araneidae.co.uk>:
>
> Running
> 	darcs get http://code.haskell.org/yi
> produces one of the following responses (depending on version).
>
> ...
> Impossible case at PatchRead.lhs:58 compiled 18:47:46 Aug 28 2007
> Please report this to bugs@darcs.net,

I'm not at all sure what the proper process for handling these bugs is (I 
see that this has created two issues, 589 and 590).  I posted a follow-up 
to darcs-devel, so hopefully my reply here is redundant (in which case, 
apologies for the extra noise): the underlying trigger of the message is 
our web proxy server quietly transforming the offending patch into 
garbage!