A door with a dial and a lever like a safe
There is a very narrow slot for a key. Insert the key and turn the dial a half rotation to the right. Pull the lever to the left. The bolt slides and the door can open.
You don't know what key to use. Try one, as above. If it doesn't work, an indicator says "please wait", the controls are locked, the key cannot be removed, and a timer starts ticking. After the timer ends, say 10 minutes, the indicator hides, the controls are unlocked, and you can try another key.
Your key is unavailable, but you know the combination. It is a 5 to 10 digit number. There are 10 positions on the dial, labeled 0, ..., 9. Turn the dial to the right as many full turns as there are digits in the combination, and stop at the first digit. Turn the dial to the left, but one fewer full turn, and stop at the second digit. After all the digits are entered, try the lever. The indicator says "please wait", the controls are locked, and a timer starts ticking. After the timer ends, if you entered the correct combination, the lever will work. Otherwise, try again.
If you cannot do any of the above and you need to get in ... break something?