Unknown vulnerability in loginwindow in Mac OS X 10.4.2 and earlier, when Fast User Switching is enabled, allows attackers to log into other accounts if they know the passwords to at least two accounts.
http://securitytracker.com/id?1014704
http://lists.apple.com/archives/security-announce/2005/Aug/msg00000.html
http://lists.apple.com/archives/security-announce/2005//Aug/msg00001.html