E.g. in aerospace the result is airplanes falling out of the sky. Seriously.
You're misinterpreting 'fail fast' - it doesn't mean 'entire system should fail catastrophically at slightest problem' or 'systems should not be fault-tolerant'. It just means that components should report failure as soon as possible so the rest of the system can handle it accordingly instead of continuing operation with an unrecognized faulty component leading to unpredictable outcomes.
In particular, "Just Sort Of Keep Going And Hope It All Works Out Eventually" is also a great way to crash planes and kill people, if we're going to pretend that everything's safety critical.
You're misinterpreting 'fail fast' - it doesn't mean 'entire system should fail catastrophically at slightest problem' or 'systems should not be fault-tolerant'. It just means that components should report failure as soon as possible so the rest of the system can handle it accordingly instead of continuing operation with an unrecognized faulty component leading to unpredictable outcomes.