According to the release notes, OS X 10.8.5 Supplemental Update 1.0 contains fixes for issues with Mail and screensaver behavior, as well as performance enhancements for AFP file transfer over 802.11ac Wi-Fi and Xsan reliability.
From the release notes:
The OS X Mountain Lion v10.8.5 Update is recommended for all users running OS X Mountain Lion v10.8.5. This update:
- Resolves an issue that may prevent certain applications from using the FaceTime HD camera on mid-2013 MacBook Air systems
- Fixes an issue that may cause external hard drvies to be ejected after the computer goes to sleep
- Addresses an issue that may prevent HDMI audio from working after waking from sleep
- Fixes an issue that may prevent certain USB Bluetooth adapters from working
As noted above, the update is recommended for all Mountain Lion users. The latest 10.8.5 supplemental update can be downloaded from Apple's