Answering Interview Question for Situation You've Never Been In?

I've tried searching but can't find a definitive answer to this. I know you're just supposed to have some stock stories to tell that can relate to a bunch of different situations, but what if they ask you one that's straight up out of left field? Getting caught off guard with one of these is my worst fear.

How do you guys go about answering these? I had one last year on HireVue where I had nothing even remotely close. So I just said "Uh, it's never happened to me, but hypothetically I would (insert some vague BS)."