Ha ha, you guys are so right about how frustrating it is that they ask this question, but in reality you're not allowed to say your "real" weaknesses because they'd sound too negative and screw you. I'm gonna take this opportunity to say my real weaknesses and vent (hopefully no ad com employees here 🙂 )
I am lazy, capital L. I will do the work I'm assigned on time, and just good enough to get an A, but I never go "above and beyond the call of duty". That's why I had so few extracurricular activities. If it were up to me, I'd just take the minimum amount of classes and no research, leadership, volunteer, or anything. The ECs I did force myself to do were only for the purpose of getting into med school. I'd rather be sleeping.
I'm really messy, poor attention to detail, and am prone to making careless mistakes. Bad memory. I can never remember peoples' names.
I'm socially inept and function poorly in a large group setting. I can be very introverted and shy, and when I try to work up the courage to talk to a stranger it often comes out awkward and stupid.
These all look really terrible, and of course I'd never say any of them in an interview. However, out of context they also paint un unrealistically negative picture of how I'd do as a doctor. Ah, now I feel better, getting these "secrets" off my chest.