I'm not a rocket scientist, but I think I can explain why this happened in a more general probabilistic sense. This error may have been "obvious", but successfu

I'm not a rocket scientist, but I think I can explain why this happened in a more general probabilistic sense. This error may have been "obvious", but successfully landing a supersonic rocket onto a floating platform probably requires hundreds or thousands of "obvious" things to go exactly right, and if any one of them doesn't, then it explodes. Even assigning a very small probability to each obvious failure mode, compounded together it gives a non-trivial chance that the landing will fail, especially in the maiden launch when there are still lots of "unknown unknowns". If it hadn't been the igniter fluid, it could just as easily have some other "obvious" thing. Anyway, that's why they do these tests: to make the "unknown unknowns" known.