I remember a couple years ago seeing this comedy bit (online) where a young couple is out at a restaurant and see this guy that they know from somewhere, but can't remember where from. During the course of the three of them trying to figure out where they know each other from, it becomes apparent that the guy is a total weirdo perv. At the very last, right as the couple is about to be seated, he says, "Oh! I remember where I know you guys from! You're Billy's parents! I'm Mr. So-and-so, his 3rd grade teacher!"

Any ideas?