Two characters have known each other for a while. The music starts and the man takes the woman by the hand, leads her to the dance floor, and proceeds to dance expertly. Stunned, she says, "You told me you couldn't dance!"
Some typical responses are:
"No, I told you I didn't like to dance."
"I didn't know I could!", or "I can't!", when he has gained some new ability via Applied Phlebotinum.
"I told you I couldn't jitterbug. I can waltz just fine," or some variant.
In one plot arc of the second season of Duel Masters, Shobu and Rekuta are surprised to find that the daughter of the nearby civilization's gatekeeper doesn't like the card game. As events proceed, it is revealed that despite her feelings, she is incredibly good at it. Her disgust stemmed from the fact that her even-more-skilled father taught her how to play, and he was her only opponent; despite her talent, she had lost every match she had ever played.
In Death Masks by Jim Butcher, Harry sweeps Susan off her feat as a means to get away from some Mooks. She says, "You told me you couldn't dance," but it turns out Harry moonlighted early in his career as a dance partner for a senior citizen organization. He can't do rock-and-roll-type dancing, but ballroom is another matter.
Harry: "Tough to tango with someone with lumbago. It requires great skill."
Along with the many, many other things he doesn't do, Rudy Miller, the hero of Gordon Korman's I Want to Go Home doesn't dance. And as with everything else, he's very, very good at it. He just doesn't do it.
In the 1998 film Dance With Me, Ruby is at first frustrated at Rafael's inability to do the fancy dance steps that she studies. Later, when she's dancing with him casually at a party, he does the step he tripped up on earlier, and reveals that he can dance quite well, but he just goes with the music and doesn't know the names of the steps.
A non-dancing example features in Quigley Down Under, regarding Quigley's apparent disdain for pistols: "I just said I never had much use for one. I didn't say I didn't know how to use it."
The Lucy Show At the beginning of Lucy and the Pool Hustler, Lucy and Mr. Moody visit a ladies' pool hall. She tells him that she doesn't like pool, and he proposes a friendly game, with a small wager. Lucy lags for break, perfectly.
Mr. Moody: "I thought you couldn't play pool?"
Lucy: "I said I didn't like it; that doesn't mean I'm not any good at it!"
In the third season of Chuck, the upgraded intersect provides several of these moments.
Casey: "I didn't know he could play flamenco guitar." Or something like that.
Cheers: In From Beer to Eternity, Woody is set up to be a bowling ace, but freezes when he tries to help the gang win a match against Gary's Old Towne Tavern. Diane (who has been making fun of bowling the whole time) steps in and wins it for them. She took bowling as a college PE class, but despises the game.
In the Doctor Who episode "The Doctor Dances", Rose is surprised to learn that the Doctor can dance, he is slightly offended that she just assumed that he couldn't dance. Before he can show her, though, they are interrupted. But they get to dance at the end with Captain Jack watching. In the Doctor Who Confidential it was said that "dancing" meant something else.
Rose: Doesn't the universe implode or something if you ... dance?
In The Big Bang Theory the girls take Sheldon to a dance and goad him to dance, assuming he would be an awkward mess. To their astonishment, Sheldon turns out to be an excelent dancer, thanks to the cotillion classes he was forced to take as a kid in Texas.
It's Your First Kiss, Charlie Brown had Charlie Brown do this... at the dance, after kissing the Little Red-Haired Girl, the rest of the evening is a blur. Linus tells him the next day that he danced with all the girls and was the life of the party, even though Charlie Brown didn't know how to dance before that.