[JohnHaugeland]

State machines are a great way to control defects by producing states which are only able to mutate in certain specific pre-defined ways

The defacto example is usually a traffic light. Green is permitted to transition to yellow, but never to red; a state machine makes a bug of that form impossible

Obviously, it's nominally used for more complex stuff, but in general, all of your appliances are state machines. Your microwave especially.