LOL! Yes, I once had someone approach me asking me all kinds of questions like 'how did your test go last Friday?' and it took me 20 minutes to realize we were colleagues (more e-mailing than seeing each other, which is why he knew a lot about me but I just couldn't remember his face)!
Funny awkward moments, I tried to ask as many generic questions as possible in the hope I would be able to remember his name :')