[kqr]

For issues that have never occurred before, probabilities are the wrong tool. The right thing to do is list all the behaviour the vehicle must never exhibit and think of ways it still might, despite all redundancies -- maybe even despite every single component working as intended.

Lots of mission failures in history were caused by unexpected interactions between fully functional components. Probabilities of failures don't help with that.

[SauntSolaire]

And why you test till failure (ideally under real/similar conditions): to surface the failures that have never occurred before, and start collecting data on them.