I notice that lightdm handles password input different than TTY (console without GUI).
try create a user with "^" character in password with English (US, with deadkeys) keyboard layout
Login using lightdm works, login via terminal within unity works. Then try it in another TTY (Ctrl+Alt+F1) and login, you will see it wont work (I have also tried it with a space bar hit after I typed the character)
Why is this and how can such persons be able to login on a TTY?