Edit: I'm noticing updates for products that don't exist on my system. Take KB2553322. That's for InfoPath, but it's not on my system.
I recently noticed Office 2010 SP2 was failing to install. It wanted me to browse to an MSI named Pro-Plus. I noticed Pro Plus was installed on the machine at some point in the past (Standard is the current version), and found where it was being referenced. Deleting the key in the registry cleared the issue, and SP2 installed.
Then the next wave of updates installed over and over. Each update says "Update is ready for downloading", but downloads instantly. According to WU log,
* All files for update were already downloaded and are valid. Deleting the
SoftwareDistribution folder forces a new download, but it doesn't fix the issue.
According to Installed Updates, under Program and Features, these updates aren't listed.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\[Your SID Here]\Products lists a few of the updates. However, one update (KB2597091) is not listed in Installed Updates or anywhere in the registry. Yet, when I try to install manually, it claims it's already installed.
Screenshot is the results of a search through the registry (location mentioned above). It is deemed installed if it mentioned anywhere. This also will give you a list of all the updates that won't install for me.