> and if you'd change tack because of the question alone, it would seem like you really lack self-confidence. Or because the candidate realized that they've me

> and if you'd change tack because of the question alone, it would seem like you really lack self-confidence. Or because the candidate realized that they've messed up, and by dropping the issue can at least salvage the next XY minutes of the interview by not going down the wrong rabbit hole. "Could you tell me more about this?" and "Are you sure about this?" are invitations for providing the rationalization for your answers. "I'm not sure that's a good idea" is a very unsubtle, but polite way of hinting that the you have gone way off the map. As an interviewer, I want my candidates to succeed. I want them to put their best foot forward. I've asked my question over a hundred times, and I've seen many ways that people have solved it, correctly or no. If I'm giving them this suggestion, it means that I know that they are going down one of the many, many wrong garden paths.