I once got a whiteboarding problem and proposed a solution, and the interviewer gave me a flat out "that's not the one I had in mind". I'd already asked clarifying questions and thought my proposal was reasonable in light of the constraints as I understood them. After a few minutes of trying to play mind reader, I asked, "Is there some constraint or requirement I'm missing?" and they said no. I retrospect, I should have walked out on that one, too. I still have no idea what they wanted from me. Communication goes both ways.