If you get a response similar to this:
[root@lxprx025 ~]# yum update
Loading "rhnplugin" plugin
Loading "installonlyn" plugin
Setting up Update Process
Setting up repositories
rhel-i386-server-5        100% |=========================| 1.4 kB    00:00
rhel-i386-server-vt-5     100% |=========================| 1.4 kB    00:00
rhn-tools-rhel-i386-serve 100% |=========================| 1.2 kB    00:00
Reading repository metadata in from local files
primary.xml.gz            100% |=========================| 1.1 MB    00:06
Traceback (most recent call last):
  File "/usr/bin/yum", line 29, in ?
    yummain.main(sys.argv[1:])
  File "/usr/share/yum-cli/yummain.py", line 102, in main
    result, resultmsgs = do()
  File "/usr/share/yum-cli/cli.py", line 359, in doCommands
    return self.yum_cli_commands[self.basecmd].doCommand(self, self.basecmd, self.extcmds)
  File "/usr/share/yum-cli/yumcommands.py", line 150, in doCommand
    return base.updatePkgs(extcmds)
  File "/usr/share/yum-cli/cli.py", line 628, in updatePkgs
    self.doRepoSetup()
  File "/usr/share/yum-cli/cli.py", line 107, in doRepoSetup
    self.doSackSetup(thisrepo=thisrepo)
  File "/usr/lib/python2.4/site-packages/yum/__init__.py", line 331, in doSackSetup
    self.repos.populateSack(which=repos)
  File "/usr/lib/python2.4/site-packages/yum/repos.py", line 203, in populateSack
    sack.populate(repo, with, callback, cacheonly)
  File "/usr/lib/python2.4/site-packages/yum/yumRepo.py", line 88, in populate
    xml = repo.getPrimaryXML()
  File "/usr/lib/python2.4/site-packages/yum/yumRepo.py", line 635, in getPrimaryXML
    return self.retrieveMD('primary')
  File "/usr/lib/python2.4/site-packages/yum/yumRepo.py", line 622, in retrieveMD
    cache=self.http_caching == 'all')
  File "/usr/lib/yum-plugins/rhnplugin.py", line 209, in _getFile
    start, end, copy_local, checkfunc, text, reget, cache)
  File "/usr/lib/yum-plugins/rhnplugin.py", line 298, in _noExceptionWrappingGet
    return result
UnboundLocalError: local variable 'result' referenced before assignment
[root@lxprx025 ~]#
try running “yum clean all” – this will remove any cached downloads from RHN and will reset any partially downloaded files.
Thanks to the guys at RedHat Technical Support for this great tip!