The Private Browsing feature in Apple Safari before 5.1.1 on Mac OS X does not properly recognize the Always value of the Block Cookies setting, which makes it easier for remote web servers to track users via a cookie.
https://exchange.xforce.ibmcloud.com/vulnerabilities/70569
http://support.apple.com/kb/HT5000
http://lists.apple.com/archives/Security-announce/2011//Oct/msg00004.html