This morning I sit in front of the Mac at work and notice that some update is required. Furthermore, the upgrade to Sierra is allowable, but they told me to not upgrade for some reason, so I simply ignore everything.
I go to the toilet for some minute and in the meanwhile the Mac start updating something. I sit again and see the white screen, the grey apple, and the progress bar which is almost full and think "Ok, this is not gonna be the Sierra upgrade, but minor updates".
The Mac reboots and...
It does not recognize the user password anymore! Ok, let's first check the caps lock key. It's off. Searching for a solution on the web I find this, and try cmd+R and everything else. In addition I set a hint, which was not present, in order to see with my eyes that the Mac has digested the change of password. I restart it. The hint is now present, but the password is unbelievably still refused!
I see a dot for each key I press, so it's out of question that some key doesn't work at all (it could work unproperly, thus giving wrong output..). Furthermore I followed the instruction of the video I linked, so I had to write
resetpassword, which proves that the keyboard is detected as QWERTY, and not AZERTY, for example.