"Oh, well you must've been rude then or I would've remembered." with a big ass smile.

"I'm sorry, you were utterly unmemorable."

"Now I remember why I chose to forget you."

I'd just call him a prick, and walk away, never to talk to him again.

Loudly: "That's an anonymous group. You know we don't talk about what goes on in there. Why couldn't you just pretend we'd never met!!"
Run off, sobbing.

That's great, seanyboy. So far it's the only response that wouldn't make me look like a jerk too...

Paging Divine Wino to etiquette thread.

I dunno; I can understand the other guy's point of view, just a little. In my line of work and way of life, I meet an insane number of random new people. Occasionally I do forget that I've already met someone. But others encounter fewer strangers in the course of a normal year, and so each one is easier to remember. It's just an imbalance, so I try to remember that not everyone has to manage a lot of work contacts and meet new people all the time. Like any social thing, renewing a contact is a learned skill. People who do interact a lot as part of a job normally never take offense when they aren't remembered by another person; they usually just say "I'm Betty from Wherever, you may remember we talked at last year's conference about your new project" etc.

It's a bit of an immature reaction, but don't take it personally. You don't have to do anything about it - it's probably clear to others that it was rude. Just say "I'm sorry. You know, I meet so many people [in this job / through Sally / doing whatever] that I sometimes get confused." Despite the other guy's breach of etiquette, you can come out looking good by having better etiquette and responding politely.

If you're trying to salvage something and stay friendly, you could say something like, "Oh, I always assume that people forget me, and it's better to err on the side of caution." But you have to stay upbeat so you don't sound like you're fishing for compliments, and you obviously can't say that after you've already asked someone where you met previously.

But in the situation you described, I'd probably say "Were you this charming then?" and write it off as a loss.

I tend to be overly polite to rude and assholish people. Bending-over-backwards-ly so. My usual response is to apologise and say that I have a horrible time remembering faces.

I've never really had anybody be rude to me about it, but then again, I usually ham up the "fuddy-duddy."

Oddest thing, the lab manager at the lab that I'm joining used to work at the same place as I did, at the same time that I did, but I can't remember her. The operation was only, like, twenty-odd people. She knew the names of my co-workers and where some of them ended up afterwards and everything.

It's absolutely bizarre (note, she didn't recognize me either; it was only after I brought up that I used to work there that she said, "Hey, I used to work there, too.")

I used to hear this shit on a nightly basis. If you need this person for something, tell them "sorry, bad with names, bad with faces." If you don't need them for anything, say something like "I meet a lot of people. I can't remember them all," and leave it at that. They'll figure it out.

Well, the Miss Manners solution would probably be a face-saving, "Oh, but I couldn't possibly have expected you to remember me."

(See how that turns it around? I'm not this good in person, I can only do this as esprit d'escalier.)

Or if you really feel a rejoinder is in order, grace of god's approach to the nines, i.e. overpoliteness, so they get the message. "I'm so sorry that you made such a slight impression on me before. That's a mistake I won't make twice." and walk away. Or an icy, "Whyever would you be concerned about that?" is the universal Ann Landers response, though.

"You know, since the concussion I have had a really hard time remembering faces. Heck I can't even remember my kids' names sometimes lately. I'm really sorry" and then grin so they can't tell if you're kidding or not.

Alternately "Oh super, so we can skip all the awkward small talk then..." and just say what you were going to say anyhow. I usually am really nice to people as if they're not being an ass and move on.

"ACTUALLY, we've MET"

"Impossible, I'd never forget such a sour odor, I'm a trained perfume counterfeiter."

Or
"My dear fellow, please forgive me, I must confess that right after meeting you I gobbled down 8 roofies and struck myself in the temple with a full bottle of Christian Brothers brandy on the off chance that our paths might cross again, so that I could greedily enjoy the sheer pleasure of encountering your radiant personality again, for the very first time!"

Or
"je suis desole, cocksucker, worse yet I'ma fucking forget you again in about five minutes, I could tattoo your face on the palm of my hand and train a parrot to sit on my shoulder and shriek your name over and over in my ear and I'd still blink and say '..and you are?' the next time we meet."

A pause, and then, "indeed". Short, and ambiguous enough.