It’s a (I assume) universal experience: Something happens, and you’re caught verbally flat-footed. A short time later the perfect rejoinder or clever joke springs to mind. What was it?
I’m taking a liberty with your OP question, but this sort of fits.
I was watching the news on Easter Sunday. They showed a photo of the Loch Ness Monster. The news was that someone had a theory that the photo is actually an erect whale penis.
I thought the newscaster missed an opportunity. He should have said, “He is risen.”
mmm
I’ll allow it. 
When he said that I have 20/20 hindsight, I should have said … (still haven’t got a comeback).
It’s over 40 years later and I still can’t come up with a good reply.
Several of us were at dinner, and I was ragging on one of the guys who was pretty laid back and mostly quiet. He had the kind of receding hairline that went up from the center of his forehead. I claimed it was because when people asked him tough questions he’d smack himself there and go “I dunno!” Dumb joke, I know, but we all laughed.
Except he sat there with a sly grin, staring at me while toying with the maraschino cherry on a stem that was in his drink. He pointedly looked at it, looked at me, and said “Trade ya…”
The whole table, including me, lost it at that point. And to this day, I can’t think of a thing I could have said to top it.
Paging @Treppenwitz. He should be the expert for this thread 
.
Soundtrack for the thread:
There’s even a wiki page for it:
“The Jerk Store called. They’re running out of you!”
Like mmm, I’m quite happy to cheat.
A few years ago a jockey called Leigh Roche rode some horse to a dead heat at The Curragh in Ireland.
I searched in vain for the headline, ‘Roche Finishes Joint First’.
Heh. I know I have many. Right now I remember none. I’ll come back if the mists clear.
j
I don’t get it 
Stare contemptuously at maraschino, smack self on hairline and say, “I dunno…why I’d like ever go for such a small stem.”
Just 40-ish years too late… 
Gotta quote Emo Philips here:
I was driving down the highway, and I’m swerving all over, coz I’m trying to change the radio, and just as I get the old one taken out I hear this traffic cop behind me, “Whee-oo, whee-oo, whee-oo!” Well, I shouldn’t make fun of his speech impediment. He asks me to walk in a straight line, so I do, then he asks me, “You call that a straight line?” Well, I should have said, I should have said, “Yes.” But I was nervous and the only thing I could think of was “Well, Officer Pythagoras, the closest you’ll ever come to a straight line is if they do an electroencephalagram of your own brainwave.”
I’m going to fudge the topic here, too, and recount the one and only time I can recall giving the “what I should have said” response.
Many, many years ago, before my first marriage, my dad (God rest his soul) took me aside one evening at the house. I was in my early 20s, and Dad was in his mid-60s.
He said, “Son, since you’re getting married, your mother thinks I should talk with you about sex.”
I said, “Sure, Dad. What do you want to know?”
He stopped for a second, chuckled, and said, “Okay then. Good talk.”
I can’t not read that in his voice.
A few years ago someone was shopping in my store for a large wicker basket (like an empty gift basket). I found one and handed it to her and she mentioned something about it being for a baby shower. The next day, another person was looking for a large wicker basket and also mentioned something about it being for a baby shower, but she specifically said a “Moses themed baby shower”. Other than it being, to me anyway, an odd theme for a baby shower, I didn’t give it much thought. However about ten minutes later I looked at a co-worker and said ‘wait, what are they planning to do with the baby?’ I still wish I had jokingly said that to her.
But seriously, that’s an odd theme, right? “Hey guys, next thursday I’m having a party. We’re pretending that I’m chucking my baby in the river”.
Kinda reminds me of The Office when Michael was watching movies at work and acting like the characters in them.
Sorry, still don’t get it
Think of “cherry” as a slang term.
Virginity?
I give up. I still don’t get it.