"If YOU don't know, we're in trouble, because I don't!" is what Evil Lotus would like to answer.
Good Lotus goes with, "I'm sorry, I don't, but I'm sure I know you. Remind me."
I meet a lot of people and, while I usually do remember people, given time, it takes a while for the associations to surface sometimes. I hate it when people do this, and always introduce myself if I think there is even a remote chance someone might not remember me. "Of COURSE I know you," is a response I sometimes get. I come back with, "How flattering!"