I've got a macbook pro running El Capitan and when I wake it up from sleep, the password works fine. However, when I power it on from being shutdown or restart it, the password no longer works. I have another admin account I can use to reset the password but I've done this twice now and it keeps happening. I'm doublechecking that the caps lock is off and that I'm not mistyping the password or accidentally pushing the shift key or something.
This link https://support.apple.com/en-au/HT202842
Talks about how to force people to enter their passwords twice in order to login. It doesn't say so, but I read into this that it's possible to have a different password for unlocking the machine and for logging on.
I also remember once having a problem where the machine was asking for both passwords even though I had autologin enabled and the passwords where the same. I think I had to reset the PRAM or something like that. Sorry, was a while ago and haven't had the problem since.
Some other links that may give you clues: