The original OS X 10.8.5 release arrived earlier this month, and it included improvements to Mail, WiFi connectivity, and screensaver functionality. The follow-up will fix some bugs that have been reported with the original release. According to the release notes (shown above), the supplemental upgrade includes fixes to FaceTime on newer MacBook Airs, USB, HDMI, and external drive connectivity. The build number is 12F42, and this is five builds newer than the original 10.8.5 release.
Apple released OS X 10.8.5 September 12th over doubling 802.11ac Wi-Fi transfer speeds.
We'll let you know when the new build is available for download.