[prof-dr-ir]

I once used a special character in my login password (on Ubuntu I think) but the keyboard settings at the passwd prompt happened to be set differently from the ones at the (graphical) login manager at boot. So in the one case, something like 'e was interpreted as é and in the other case it wasn't.

I think it took me about five reboots in single-user mode and password resets before something clicked. I wish Ubuntu would not have allowed special characters. :)