I've had a handful turn up who I remember seeing before yet they've behaved as if they've never clapped eyes on me in their life, and I've had to give directions as usual too. I honestly don't know whether they've forgotten or not but I've never let on.
To be fair, I've done it too! They can tell me they've been before and come straight to my building on the day, and still when I see them I'd swear on a stack of bibles I've never met them before in my life. Punter blur.