This is simple to reproduce by editing lightdm.conf and setting greeter-session= to something made-up, like greeter-session=foo and then restarting lightdm with sudo initctl restart lightdm.
This is simple to reproduce by editing lightdm.conf and setting greeter-session= to something made-up, like greeter-session=foo and then restarting lightdm with sudo initctl restart lightdm.