The error shows up in the log but the API call stills produces a successful return. 

After n number of sync calls:

==> pulp.log <==
2011-04-05 09:19:24,702 [INFO][Thread-1606] sync() @ repo_sync.py:487 - YumSynchronizer reported 7 successes, 0 downloads, 0 errors
2011-04-05 09:19:24,710 [ERROR][Thread-1606] failed() @ task.py:207 - Task id:52ece4e8-5f87-11e0-8a6f-545200104e49, method_name:_sync:
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/pulp/server/tasking/task.py", line 131, in run
    result = self.callable(*self.args, **self.kwargs)
  File "/usr/lib/python2.7/site-packages/pulp/server/api/repo.py", line 1491, in _sync
    threads)
  File "/usr/lib/python2.7/site-packages/pulp/server/api/repo_sync.py", line 121, in sync
    return synchronizer.add_packages_from_dir(repo_dir, repo, skip_dict)
  File "/usr/lib/python2.7/site-packages/pulp/server/api/repo_sync.py", line 231, in add_packages_from_dir
    package_list = pulp.server.util.get_repo_packages(dir)
  File "/usr/lib/python2.7/site-packages/pulp/server/util.py", line 214, in get_repo_packages
    r.sack.populate(r, 'metadata', None, 0)
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 163, in populate
    if self._check_db_version(repo, mydbtype):
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 221, in _check_db_version
    return repo._check_db_version(mdtype)
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 1224, in _check_db_version
    repoXML = self.repoXML
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 1413, in <lambda>
    repoXML = property(fget=lambda self: self._getRepoXML(),
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 1405, in _getRepoXML
    self._loadRepoXML(text=self)
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 1395, in _loadRepoXML
    return self._groupLoadRepoXML(text, self._mdpolicy2mdtypes())
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 1370, in _groupLoadRepoXML
    if self._commonLoadRepoXML(text):
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 1198, in _commonLoadRepoXML
    result = self._getFileRepoXML(local, text)
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 985, in _getFileRepoXML
    size=102400) # setting max size as 100K
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 808, in _getFile
    size=size
  File "/usr/lib/python2.7/site-packages/urlgrabber/mirror.py", line 408, in urlgrab
    return self._mirror_try(func, url, kw)
  File "/usr/lib/python2.7/site-packages/urlgrabber/mirror.py", line 394, in _mirror_try
    return func_ref( *(fullurl,), **kwargs )
  File "/usr/lib/python2.7/site-packages/urlgrabber/grabber.py", line 982, in urlgrab
    return self._retry(opts, retryfunc, url, filename)
  File "/usr/lib/python2.7/site-packages/urlgrabber/grabber.py", line 886, in _retry
    r = apply(func, (opts,) + args, {})
  File "/usr/lib/python2.7/site-packages/urlgrabber/grabber.py", line 968, in retryfunc
    fo = PyCurlFileObject(url, filename, opts)
  File "/usr/lib/python2.7/site-packages/urlgrabber/grabber.py", line 1063, in __init__
    self._do_open()
  File "/usr/lib/python2.7/site-packages/urlgrabber/grabber.py", line 1350, in _do_open
    self._set_opts()
  File "/usr/lib/python2.7/site-packages/urlgrabber/grabber.py", line 1161, in _set_opts
    self.curl_obj.setopt(pycurl.NOPROGRESS, False)
error: cannot invoke setopt() - perform() is currently running

==> grinder.log <==
2011-04-05 09:19:24,712 [ERROR][Thread-1638] getRepoData() @ RepoFetch.py:153 - Unable to Fetch Repo data file filelists
2011-04-05 09:19:24,714 [ERROR][Thread-1638] getRepoData() @ RepoFetch.py:153 - Unable to Fetch Repo data file updateinfo
2011-04-05 09:19:24,717 [ERROR][Thread-1638] getRepoData() @ RepoFetch.py:153 - Unable to Fetch Repo data file other
2011-04-05 09:19:24,717 [ERROR][Thread-1638] getRepoData() @ RepoFetch.py:153 - Unable to Fetch Repo data file primary

==> pulp.log <==
2011-04-05 09:19:24,718 [ERROR][Thread-1638] failed() @ task.py:207 - Task id:5379de2e-5f87-11e0-8e0b-545200104e49, method_name:_sync:
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/pulp/server/tasking/task.py", line 131, in run
    result = self.callable(*self.args, **self.kwargs)
  File "/usr/lib/python2.7/site-packages/pulp/server/api/repo.py", line 1491, in _sync
    threads)
  File "/usr/lib/python2.7/site-packages/pulp/server/api/repo_sync.py", line 117, in sync
    progress_callback, max_speed, threads)
  File "/usr/lib/python2.7/site-packages/pulp/server/api/repo_sync.py", line 484, in sync
    report = self.yum_repo_grinder.fetchYumRepo(store_path, callback=progress_callback)
  File "/usr/lib/python2.7/site-packages/grinder/RepoFetch.py", line 355, in fetchYumRepo
    self.yumFetch.getRepoData()
  File "/usr/lib/python2.7/site-packages/grinder/RepoFetch.py", line 157, in getRepoData
    shutil.copyfile(src, dst)
  File "/usr/lib64/python2.7/shutil.py", line 82, in copyfile
    with open(dst, 'wb') as fdst:
IOError: [Errno 2] No such file or directory: u'/var/lib/pulp//repos/pub/updates//repodata.new/repomd.xml'