The problem statement was straight-forward; I jumped right in, covered all the cases (which I guess made the interviewers think I mugged up the solution; could tell from their reaction). When there was ~5 mins left, I was asked to cover another case which was not a part of the initial requirements. You know how hard is it to ideate a new requirement when you know that you have just 5 mins left and had already coded in a certain way. Very bad experience.