Bug #1463112 “Cat sitting on keyboard crashes lightdm” : Bugs : unity package : Ubuntu
bugs.launchpad.net
14.04, locked screen to go to lunch, upon return from lunch cat was sitting on keyboard, login screen was frozen & unresponsive. To replicate: In unity hit ctrl-alt-l, place keyboard on chair. Sit on keyboard. Resolution: Switched to virtual terminal, restarted lightdm, lost all open windows in X session. What should have happened: lightdm not becoming unresponsive.

Found the story behind this bug neat, that is why I wanted to share it.