The combination lock is a popular house design. If done right can only defeated by brute force, making it just about unbreakable. It's also very boring and tedious to crack.
The combination lock presents the burglar with any number of switches (usually 10 or more). Triggering exactly the right ones connects a circuit, which allows access to the vault.
A simple and crude example is in the screenshot.
The power source is connected through a voltage-triggered switch to a powered door, forcing it shut.
Stepping on the right pressure switches (in this example, the first and third switches) trips the voltage switch, breaking the connection between the power source and the door.
The only way to solve it would be try every possible permutation of the switches, checking after each trigger if the trap has opened. In this example there are 6 permutations: 123, 132, 231, 213, 312, 321.
Both 132 and 312 will open the door, however the player must not step on '2' or the door closes again. This means intruders must trek back to the door (if it's our of view) after every switch is triggered, making it incredibly tedious. For an even more tedious lock, see Pattern Lock.