My sister is the great comeback artist in the family.
My dad: What’s the name of that tube that connects your ear and your throat? The Fallopian tube?
My mother: That’s your eustacian tube, you dope.
My sister: Whaddaya want, Dad, eggs in your ear?
snort
I had to have some dental work done and the dentists says to me “Now bite down on this, we need to take an impression.”
I snaps back “I don’t do impressions.”
Over xmas my sis was wearing these ‘wind pants’, like polyester that are rather loud when you walk or move at all. So most of the family is sitting around, and she walks by, yells at her kids to turn the volume down on the tv, and starts to walk out. The noise from her pants is noticeable. I say "Could you turn the volume on those pants down a little. " My family likes to tease, too much, and that line was repeated every opportunity.
Today, after a gut-wrenching week of computer training the trainer said we were going to get off early-3:30!!! (a whole hour early, and 30 minutes later than when I usually get off). It was a very difficult training course, and there were computer problems, and other things like that. My co-worker sitting next to me (and ex-girlfriend) said sarcastically "What are you going to do with your time off? I said “Drink heavily”.
“F@&k you, clown!”
OK, I was in the band for this staged reading of a new musical.
We rehearsed a couple of weeks, did a show, rehearsed again, did a show, packed it all up and set up in LA–and the director then announced we would rehearse yet again before the show that evening.
At that point, the guitarist opined that you can’t polish a turd.
Whereupon I said “Hmph! Obviously you haven’t seen my turd collection!”
I have to admit, I was pretty happy with that, and it really did just come out of my mouth with no thought whatsoever.
Unfortunately, it was memorable enough that everyone who heard it continues to associate me with the phrase turd collection, which is perhaps a less than ideal outcome.
My brother has become famous in his office for his off-the-cuff remarks. One day his supervisor wanted to know how his work was going:
Supervisor: “Where do you stand?”
Brother: “Alone. In the shadows. Ever vigilant on a quest for justice.”
Supervisor: “How’s your workload?”
Brother (brightly): “Good.”
I’m borrowing that one at the first opportunity. 
The closest I ever came to this: me in college 15 years ago, I run into Joe. Joe’s carrying a breakfast pastry. Thinking as fast as I ever would:
“Hey Joe, where you goin’ with that bun in your hand?”
Sleeping in on a Sunday morning, my husband and I are awakened by our 4 year old daughter pounding on our door, yelling “I know what you’re doing in there and it’s not right!!”
I turned to my husband and said “See. I told you you weren’t doing it right.”
Got into it with the quack who did my eyelid surgery. Picture a “Doc Hollywood” type who looks a lot like Moe Greene in the first Godfather movie. He’s obviously pissed that I am not happy with the lousy job he did on my face. (He thinks a “50% improvement” is “excellent.”)
Finally, after I’ve explained to him for the third time why I am not happy with the work he did, he stands up and says “That’s it. We’re done here. I think you had *excellent *surgery.”
“If this is your standard of excellence,” I shot back at him before he flounced out the door, “it sucks.”
Not the best comeback I’ve ever launched, but it was very satisfying.
I got an “invitation” like that once. I said “Can you hang on a sec? I need to see if my husband’s asleep yet. He just got home from the night shift on the vice squad.”
I think the guy melted the phone hanging up that fast…
Speaking of jockeys… my son’s 6’7" and has long since gotten a wee bit weary of people asking him if he plays basketball.
So when he’s asked, he says “No, I’m a jockey.”
A friend once followed that up with “On the Clydesdales.”
Not the last but one of my favourites. A week or so after my girlfriend dumped me, we met in a pub so that she would return me some books I had lent her. There were also some of my friends sitting at the table. She handed me the books and said in a cold voice: “Do I still have something of yours?”
To which I spur-of-the-moment-ly replied with: “Well, if you got your period, you don’t.”
This happened about 4 years ago and after a year of icy looks we befriended again and are really good friends now.
Great! I was able to pull off this line, after having to wait 40-some years for the right circumstances: “Pardon me, Roy, is that the cat who chewed your new shoes?”