Pushing to a local git repo using hg-git, I got the following output:

(actually some of the "garbage" characters didn't pass through the coding system correctly, so you can download the raw output here)

Specific steps to reproduce:

1. Install mercurial and hg-git
2. hg clone https://dabrahams@bitbucket.org/dabrahams/pip/
3. mkdir pip-git; cd pip-git; git init; cd ..
4. cd pip
5. hg push ../pip-git

pushing to /Users/dave/src/pip-git2
importing Hg objects into Git
creating and sending data
** unknown exception encountered, details follow
** report bug details to http://mercurial.selenic.com/bts/
** or mercurial@selenic.com
** Mercurial Distributed SCM (version 1.5.1)
** Extensions loaded: bookmarks, graphlog, hggit
Traceback (most recent call last):
  File "/opt/local/bin/hg", line 27, in 
    mercurial.dispatch.run()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/mercurial/dispatch.py", line 16, in run
    sys.exit(dispatch(sys.argv[1:]))
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/mercurial/dispatch.py", line 30, in dispatch
    return _runcatch(u, args)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/mercurial/dispatch.py", line 47, in _runcatch
    return _dispatch(ui, args)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/mercurial/dispatch.py", line 467, in _dispatch
    return runcommand(lui, repo, cmd, fullargs, ui, options, d)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/mercurial/dispatch.py", line 337, in runcommand
    ret = _runcommand(ui, options, cmd, d)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/mercurial/dispatch.py", line 518, in _runcommand
    return checkargs()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/mercurial/dispatch.py", line 472, in checkargs
    return cmdfunc()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/mercurial/dispatch.py", line 466, in 
    d = lambda: util.checksignature(func)(ui, *args, **cmdoptions)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/mercurial/util.py", line 401, in check
    return func(*args, **kwargs)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/mercurial/commands.py", line 2445, in push
    r = repo.push(other, opts.get('force'), revs=revs)
  File "/Users/dave/src/hg-git/hggit/hgrepo.py", line 19, in push
    git.push(remote.path, revs, force)
  File "/Users/dave/src/hg-git/hggit/git_handler.py", line 147, in push
    changed_refs = self.upload_pack(remote, revs, force)
  File "/Users/dave/src/hg-git/hggit/git_handler.py", line 545, in upload_pack
    changed_refs = client.send_pack(path, changed, genpack)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/dulwich-0.5.1-py2.6-macosx-10.6-x86_64.egg/dulwich/client.py", line 269, in send_pack
    return client.send_pack(path, changed_refs, generate_pack_contents)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/dulwich-0.5.1-py2.6-macosx-10.6-x86_64.egg/dulwich/client.py", line 131, in send_pack
    raise ChecksumMismatch(sha, client_sha)
dulwich.errors.ChecksumMismatch: Checksum mismatch: Expected (�B‘Ú§eNçhÚØ�˙h’�@"–, got �error: refusing to update checked out branch: refs/heads/master