There appears to be an issue with the App Store update from 10.8.4 to 10.8.5 typically called a delta update. Downloading and running Apple’s 10.8.5 Combo Updater which restarts your Mac and puts it right back to 10.8.5 makes the issue vanish.
Combo updates are Apple’s version of their latest patches and also include all previous updates for that operating system. Delta updates only include the patches since the last update version.