I said "well first, this is a mess, I'm putting parentheses here, here, here and here". They said "well you've fixed the bug but can you tell us where it was?"
I gave them a hypothesis but I said my "real answer" was that it's not worth our brain cycles to figure it out, you just shouldn't write code that requires knowing operator precedence. It's just such desperately boring information that I can't hold it in my head.
Interviewing such an insufferable smartarse was probably quite annoying but they did give me the job and I do stand by the underlying principle!