Question: **Background**

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...

**The problem**

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](https://www.youtube.com/watch?v=CJ6AZMk2cy0), 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!

**EDIT**:

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 `Q``W``E``R``T`‌​`Y`, and not `A``Z``E``R``T`‌​`Y`, for example.

Answer: 
In the end I found no reasonable answer to my question. So I decided to set the password (with the method mentioned in the question) to void. Then when the mac is boot the password in no more asked, thus solving the problem.
Thanks for the post. Just discovered an old iMac which had exactly the same issues as (very nicely) described by yourself.

I think the issue was to do with the update removing wifi connection passwords rather than anything else. If you CMD+R and enter the recovery mode, you can click on the wifi icon and re-enter the wifi connection password. It will all work then too without need to create / edit passwords etc

Thanks Again