Error from smart server: ScopeReplacer object '_KnitGraphIndex' was used incorrectly

The Launchpad branch puller recorded this error message as OOPS-1229SMPM21.

I spoke to Robert about it on IRC, and he suggested that it was a bug in Bazaar.

UnknownErrorFromSmartServer: Server sent an unexpected error: ('error', "ScopeReplacer object '_KnitGraphIndex' was used incorrectly: Object already cleaned up, did you assign it to another variable?: _factory")

Traceback (most recent call last):
  (elided...)
  Module bzrlib.branch, line 135, in open
    return control.open_branch(_unsupported)
  Module bzrlib.remote, line 287, in open_branch
    return RemoteBranch(self, self.find_repository(), format=format,
  Module bzrlib.bzrdir, line 649, in find_repository
    found_repo = self._find_containing(usable_repository)
  Module bzrlib.bzrdir, line 615, in _find_containing
    result, stop = evaluate(found_bzrdir)
  Module bzrlib.bzrdir, line 639, in usable_repository
    repository = found_bzrdir.open_repository()
  Module bzrlib.remote, line 332, in open_repository
    response, real_repo = probe(path)
  Module bzrlib.remote, line 304, in _open_repo_v2
    response = self._call(verb, path)
  Module bzrlib.remote, line 57, in _call
    self._translate_error(err, **err_context)
  Module bzrlib.remote, line 129, in _translate_error
    _translate_error(err, bzrdir=self, **context)
  Module bzrlib.remote, line 2563, in _translate_error
    raise errors.UnknownErrorFromSmartServer(err)
UnknownErrorFromSmartServer: Server sent an unexpected error: ('error', "ScopeReplacer object '_KnitGraphIndex' was used incorrectly: Object already cleaned up, did you assign it to another variable?: _factory")