Asking how it will fail is a gotcha? To my knowledge and experience thus far, all systems fail. The question is "given this one, that we now have a good shared understanding of, which you designed with the tools you are most familiar with, how is it going to fail?

Not one candidate I can think of has been either surprised by this question, or felt stuck, fwiw. Again, I may be completely self delusional, but I hope not!