The 10.8.5 update is recommended for all OS X Mountain Lion users and improves the stability, compatibility, and security of your Mac. This update:

Fixes an issue that may prevent Mail from displaying messages

Improves AFP file transfer performance over 802.11ac Wi-Fi

Resolves an issue that may prevent a screen saver from starting automatically

Improves Xsan reliability

For detailed information about the security content of this update, please visit: http://support.apple.com/kb/HT1222