Screen Lock in Apple Mac OS X before 10.8.5 does not properly track sessions, which allows remote authenticated users to bypass locking by leveraging screen-sharing access.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2013-1073
http://support.apple.com/kb/HT5880
http://lists.apple.com/archives/security-announce/2013/Sep/msg00002.html