The kernel in Apple Mac OS X before 10.7.2 does not properly implement the sticky bit for directories, which might allow local users to bypass intended permissions and delete files via an unlink system call.
http://www.securityfocus.com/bid/50085
http://support.apple.com/kb/HT5002
http://lists.apple.com/archives/Security-announce/2011//Oct/msg00003.html