Chu Chu, what you are missing is the "fail-safe" philosophy. It is integral to the design of aircraft systems and a requirement for certification.
Simply meaning that if anything breaks / faults, the resulting state may be a limited functionality, but strictly not that of an unpredictable behaviour. The failure modes (cases) are thorougly investigated at the design stage.
For instance the nose wheel steering above. Normally it blends inputs from rudder pedals and the two dedicated tillers to command nose-wheel deflections. The ratio is dependent on airspeed. If the speed measurment is suspect or wrong - by design then - the value of speed will be disregarded up to the point when nose-wheel steering is made unavailable. (there is some dampened free-castor, so afterwards you still can maintain some directional authority through assymetrical braking). True, when the system is unable to self-diagnose the errorneous reading, you may need to turn the Air Data System unit off, that is all to it.