This question comes up often and I really don't see the point. I have sat on the other side of the table many times and most interviewees answer this in two ways, either completely sidestepping or giving a minor point and then turning it positive. It really is not helpful to either party in my opinion; if you wish to know the individual's weaknesses, speak to their former colleagues/manager. Even as a test of "self-recognition" or "honestly" it is setup for failure, given the whole intent of the interviewee to put on their "best face."