I've noticed that quite often Red Hat's yum will fail for no good reason. For example it will just show an error like:
# yum update
Error Downloading Packages:
samba-common - 3.0.28-1.el5_2.1.x86_64: failed to retrieve getPackage/samba-common-3.0.28-1.el5_2.1.x86_64.rpm from rhel-x86_64-server-5
error was [Errno 14] HTTP Error 404: Not Found
When that's the case, retrying a few times usually does the trick.
I have actually opened a ticket with RHN's support and they've told me these are "temporary errors". Temporary or not they are pretty annoying so here's what I do.
1. Retry manually until you're happy with the list of packages that will be updated
2. Run yum in a while loop so that it retries until it succeeds:
# while ! (yum update -y) ; do echo TRY AGAIN; done
Be aware that if that too many retries can cause your RHN subscription to be temporarily locked-out. I find the benefit too great to care.