Not in real life, but on the Internet this happens a lot. I speak to lots of people casually (through websites and irc) and sometimes someone will be all "hey citric! Do you remember me?" and I'm always like "oh shit!". I generally just say "uh, I think so, are you that guy from ..." and use some ridiculously general place so if I'm right they elaborate themselves, if I'm wrong (which is never) then I don't do anything because it's never happened.
I'll also be sneaky and google their username, but shhh.