Completely and totally random hypothetical scenario:
Say you’ve got this friend. You’re not best buddies, but say you work together, maybe sometimes go out for a beer after work, etc. Said friend one day invites you to a party he’s having (Say, New Years or something. Not a BigHugeDeal). You say you’ll be there. When you arrive, you discover that one of your friends’ other buddies is someone famous. Not ‘has some name recognition’, but Really Freakin’ Famous: Bill Gates, Brad Pitt, cover-of-People-magazine-famous.
A: Acknowledge that, since they’re very famous, they’re used to being recognized, and it would be stupid to act like you have no clue who they are, or
B: Ignore the fact that they are famous, as you’ve never personally met them before, and go through the whole introduction thing as if you don’t know who they are?