We use a cash register from the wild west. It's a very simple keypad because it have only 6 button: 3x "1" and 3x "2". So we want a door to open if the player press 2-2-1. In this order. Otherwise it's too easy.
So we created a lock and we link the first "2" and second "2" and third "1" to the lock what opens the door. But if the player press the buttons randomly or even he press every button sequently 1-1-1-2-2-2 it also open the door.
So we link all the "not okay" buttons to the lock "reset" field, so if the player press any not okay button, it will reset the whole lock, so player should press only the correct buttons, otherwise (if he press only one wrong button) it reset the whole lock.
But unfortunatelly if he press all buttons in sequently, he will press the okey buttons too: 1-1-(1)-(2)-(2)-2 and the door open. So we need some logical way to open the door only if he press the buttons in ORDER. Now the 1-2-2 is also works, because the order doeast important in the lock.