Here's something you GMs can do with your players. Have some random NPC walk up to the PCs and casually start a conversation with them. It's clear that the NPC knows them, but the players have no idea who the NPC is. Watch them try to handle the situation smoothly, or better yet, fail to handle the situation smoothly.

The NPC can be someone they really did meet a while back, but forgot about because they were generically anonymous, like some random shopkeeper or something. Or it could be someone they know in disguise, or who has otherwise changed appearance somehow. Or - the most intriguing option - it could be someone pretending to know the PCs for some reason. Whatever the reason is, you can be sure it's something no good. And that makes it even more hilarious when the players stumble through pretending they remember this person they've really never met before. For bonus fun have it be someone obviously important, like a noble or something.

Transcript

GM: A female pig enters, in a white robe and with hair buns.
Luke: Another one?
Miss Piggy: Hey, R2.
Miss Piggy: Hey, Luke.
Luke: Do I know you?
Miss Piggy: Got your back, good buddy.
Luke: Wedge???
Miss Piggy: The one and only.
Luke: Wait, are you a clone of Wedge?
Miss Piggy: Do I look like a clone of Wedge?
Luke: Heck, I dunno what Wedge looks like any more.