Don't worry - that is what that question is supposed to do. It is supposed to make you feel uncomfortable and unsure. It is supposed to throw you off. Interviewers generally ask this question as a way to find out how you handle yourself in an uncomfortable situation. They want to know if you can manage yourself, even when feeling uncertain.
So how do you handle it?
Check out the Good, the Bad, and the Ugly for some DOs and DON'Ts: