Failed Sensor Delays Shuttle Launching - New York Times
The sensor in the hydrogen fuel tank is designed to ensure that the spacecraft's main engines run for the correct amount of time after liftoff. There are three other sensors in the tank, and NASA could have decided to fly with one malfunctioning, but instead chose to adhere strictly to its safety standards.
It just takes two major disasters in a hundred or two flights as a result of ignoring the safety criteria and design.
Remember, safety factors exist so that during the flight/deployment/whatever, when the unexpected happens you can still survive. Most of the time, redundant systems are not redundant so that you can "get by" on less.
Richard Feynman would be proud.
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