Ran into an issue while recently patching a SharePoint 2010 farm from Service Pack 1 to Service Pack 2. Patching is a bit of a science and can be complex depending the number of servers and products installed in your farm. I applied Service Pack 2 and December 2013 Cumulative Update, KB2553400, for SharePoint and Office Web Applications, 4 total updates.
Note - I have found where customers have SharePoint and Office Web Apps on different patch levels or worse Office Web Apps has never been patched. It's best practice to have SharePoint and Office Web Apps at the same patch level.
All 4 patches installed without error. At least the install wizard said they did. 
I ran psconfig.exe, aka SharePoint Products Configuration Wizard, and boom "Error: Some farm products and patches were not detected on this or other servers. If products or patches are missing locally, you must quit this program and install the required products and patches on this server before restarting this wizard." SharePoint thinks the issue is that one of the servers is missing the Office Web Apps December 2013 Cumulative update.
Full list of updates
The kicker, these updates were installed and if you are wondering, no we did not run into this issue when we patched our test Farm. I verified these patches were installed in Programs and Features, recently installed updates. Restarted each SharePoint server after the updates. Still SharePoint thought we hadn't patched one of the servers.
I remembered running into a similar issue while applying updates on another large SharePoint Farm that had 10 plus language packs. Good Times!
Resolution
- From SharePoint Shell, run the 2 commands below on each SharePoint server.
- Get-SpProduct - -Local
- (Get-SpServer $env:ComputerName).NeedsUpgrade
- Confirm "True" is returned
Continue with running the SharePoint Products Configuration Wizard
SharePoint 2010 Patches
Office 2010 Web Apps Patches
Thanks!
Matt Engibous