Power Management in Apple Mac OS X before 10.8.5 does not properly perform locking upon occurrences of a power assertion, which allows physically proximate attackers to bypass intended access restrictions by visiting an unattended workstation on which a locking failure had prevented the startup of the screen saver.
http://support.apple.com/kb/HT5880
http://lists.apple.com/archives/security-announce/2013/Sep/msg00002.html