This question is a stress test! The interviewers want to know if you are happy with your performance in the interview. There is always room for improvement; however, you want to avoid picking your interview apart.

If you feel you could have performed better, you can answer by saying something like: 'I feel that I was well prepared for most of the questions you have asked me today; however, I would like to clarify my answer on one particular point....' Then, return to the question you may have stumbled on and answer how you would have liked the first time.