Agree that this isn't a reasonable "normal" programming mistake to run into, but I think the question may be used for seeing how candidates approach a problem that they don't know how to solve.

I doubt the interviewers are looking for someone to get this question 100%, and more likely looking for how candidates would deal with a problem they haven't been confronted with before that seems unsolvable. Does the candidate just say it's impossible or a trick? Or do they double down and try to think of what could possibly cause this and offer up some possibilities?