Backdoor #4: Ctrl-Alt-KP_Multiply. This keystroke kills any X11 app that holds a lock, so typing this will kill xscreensaver and unlock the screen. This "feature" showed up in the X server in 2008, and as of 2011, some vendors are shipping it turned on by default. How nice. You can disable it by turning off AllowClosedownGrabs in xorg.conf.