I've installed service pack 1 for MOSS numerous times now and has been pretty painless, until one install on a client site last week.
As usual I ran the wss 3.0 sp install, cancelled the config wizard, ran the MOSS sp install and then let the config wizard run.
The trouble started when it got to step 8 of 9 and failed.
The error was reported as "An exception of type Microsoft.SharePoint.PostSetupConfiguration.PostSetupConfigurationTaskException was thrown. Additional exception information: Failed to upgrade SharePoint Products and Technologies."
I did a quick search and found this article http://technet2.microsoft.com/windowsserver/WSS/en/library/91649a7e-6b5a-4e5a-9ee5-51951f4b857f1033.mspx?mfr=true , tried the fix suggested to no avail.
This is when I started to get a little worried, this was a live production server and the service pack had brought it completely down, plus there is no way to uninstall it so it was pretty much fix or rebuild. I rang MS support who ever told me that it would be 48 hours before a SharePoint expert could ring back as they had a big queue of SharePoint problems which was not good news.
After a bit more searching I found you can actually run the upgrade at the command line.
It's a stsadm switch "stsadm -o upgrade -inplace -url http://intranet " this is heaps better than running the wizard as it actually gave quite details error messages in the log file. This is definitely a good thing to try if you are having trouble with the service pack.Turns out the problem was the sys admin had setup a dr server and was replicating the database out to it. This locked the schema from being modified. After deleting the replication and log shipping settings the upgrade worked :)
I'm still trying to find out if there is a better way to run a sp/hotfix on SharePoint with replication setup but so far MS support haven't given me an answer.