Just installed the Oktober patch for SharePoint server 2013. http://support.microsoft.com/kb/2825647
I knew that after installing a patch you should always run the SharePoint Products 2013 Configuration Wizard so I did. However the patch still left me with a “HTTP Error 503. The service is unavailable.” error.
Solving this is rather easy, SharePoint patches have the tendency of shutting down the IIS application pools. So all you have to do is start up IIS manager navigate to the pools and hit the ‘play’ button.
Here you can see that the installation of the patch stops all pools but doesn’t start them back up again after finishing the installation.