The installation of SharePoint 2013 CU201308 was fine. But when I ran the SharePoint 2013 Products Configuration Wizard on the first web front end server, I got error message complaining that Hotfix KB2817616 was missing on one of the servers.
I thought I missed that server, but when I tried to install SharePoint 2013 CU201308 on that server, it said "There are no products affected by this package installed on this system".
I tried to Clearing the Configuration Cache, restarted the timer service, but nothing worked.
In terms of Cumulative Patch installation rule, there is no way to uninstall or re-install a CU patch. So I stuck there.
So this server have to be sacrificed to this CU installation. I disconnected this server from the farm, and then upgraded the farm successfully.
It won't hurt to give it a last chance. After join the server back to the farm, it's upgraded without any problem!
No sure what's the problem, but anyway, it works now.
We can disconnect the server from the farm through PowerShell command "Disconnect-SPConfigurationDatabase –Confirm:$false", reference: http://technet.microsoft.com/en-us/library/ff607702(v=office.14).aspx