The screen saver in MacOS X allows users with physical access to cause the screen saver to crash and gain access to the underlying session via a large number of characters in the password field, possibly triggering a buffer overflow.
http://docs.info.apple.com/article.html?artnum=120232
http://archives.neohapsis.com/archives/bugtraq/2003-07/0187.html
http://archives.neohapsis.com/archives/bugtraq/2003-07/0034.html