Revenge of the Nerd

An unbelievably hot woman appears to seduce the protagonist, but ends up humiliating and rejecting him. He then finds out she is the acne-ridden wallflower he ridiculed in High School -- all grown up, but still bitter.

If she's really pissed off, this plot can morph into Reunion Revenge.

Compare Something's Different About You Now, where he hits on her... only to find out she used to be a "he".

Not often seen gender-reversed, but it's not unheard of.

Examples:

Comic Books

 * Empowered gives a gender-flipped example. Most superheroes in the setting are assholes with undeserved praise heaped upon them. At a superhero award show, the heroes nominated an unpopular, D-list superhero for a major award, only to humiliate him onstage. The guy used his shapeshifting powers to create a new identity for himself, and his new persona let him become a respected superhero. At first he was content with his new life, until he learned that heroes who humiliated him were planning to pull the same trick on the protagonist. As a result, he snapped and attempted to kill everyone during the ceremony.

Film

 * The '70s TV movie The Girl Most Likely To has Stockard Channing as a campus ugly duckling who gets in a car accident, and subsequently undergoes plastic surgery that turns her into a knockout. She spends the rest of the movie finding the pretty people who'd picked on her before the accident and bumping them off one by one.

Live Action Television

 * The Drew Carey Show has flipped example where Oswald, who was likeable enough in high school, had dumped a girl because she was extremely fat and never talked to her again after the second date (he specifically goes out of his way to avoid her). As an adult the same girl, now 140 pounds lighter, gets him to a agree to be handcuffed on the bed where all the guests coats are for a party. She then pulls his underwear down and tells him who she was; she actually told him her name at the beginning but Oswald had only remembered her as "porky" the fat girl
 * Gender-reversed: Donny Doyle in 8 Simple Rules
 * In one episode of Fantasy Island, Adrienne Barbeau played an overweight woman whose fantasy was to become thin and beautiful so that she could seduce the man who humiliated her in high school (who also happened to be on the island) and get revenge on him.
 * Friends, in at least one episode. Chandler has always been a Man Child, apparently since he was a child, and there was one incident in high school where he pantsed a girl during a school play or something. When they happen to bump into each other as adults she's played by Julia Roberts. Since at this point in the series Chandler is an Unlucky In Love Butt Monkey, he's grateful for the attention from someone so hot, and very grateful that she seems to have forgiven him. In fact, he's so grateful for it that he goes along with it when she expresses an interest in him wearing woman's underwear for a day, and then having sex in a bathroom stall. Of course, she takes his clothes and leaves. Turns out she hadn't forgiven him.
 * There was also the Thanksgiving episode where it revealed that a now slimmed down Monica attempted to do this to Chandler in revenge for him calling her fat the previous Thanksgiving. Monica hilariously tries and fails to act aroused by everything leaving Chandler more confused than anything else.
 * Completly averted in Ghost Whisperer. Jim is being haunted by the ghost of a girl he knew in highschool. She was a supermodel before she died, but the ugly duckling in school. The twist is she's trying to comunicate with him because he was the only guy who ever made her feel pretty back before all the plastic surgery. And she wants him to save her sister from the same fate she suffered.
 * Growing Pains
 * A one-shot antagonist (protagonist?) in Its Always Sunny in Philadelphia was getting engaged to and then breaking it off with everyone who ever turned him down in high school due to his acne problem, including, and had been planning on going after Dee next. It is unknown if Charlie detered him, because we never hear of it again.
 * Married With Children inverted by having Bud (the protagonist/nerd) winning over the girl for the purpose of humiliating her (usually it is the guess character who wants revenge), also inverted in the sense that it is gender-reversed. Then it is averted by having Bud relent at the last moment because he falls for the girl again.  Bud might be justified in seeking revenge because they are still close to the humiliating event (7th grade vs high school) and people still make fun of him (one of those things that happens frequently but we only see once). The pure trope involves the avenger holding on to her pain for years, sometimes even decades.
 * She didn't get off scott free anyway. Kelly, figuring this would happened, manged to bind and gag her, dress her in nothing but a towel and then tie a string to Buck's mouth while waiting for morning classes in the school. She then called Buck over and had him tug off the towel just as kids were filling the halls.
 * There was another example from Married With Children and a gender-reversed one. This time it was Kelly who played the part of the bitch, standing up a nerd late at night outside in the middle of winter so that he lost some toes to frostbite. Years later, Kelly attends a class reunion only to find that the nerd has become a handsome millionaire whom Kelly and all of her Alpha Bitch friends fall for. The former nerd enacts his revenge by standing Kelly up on a date out in the cold and by arranging for Bud to secretly take his place on a date under the bleachers with one of the other bitchy girls. Oh, and then once that date is over, a nerdy girl whom Bud had blown off earlier that night traps him under the bleachers and starts having her way with him.
 * Sports Night: Bobbi Bernstein is a substitute anchor who rails against Dan for having slept with her in Spain and then never called her. Dan claims to have never met Bobbi before she started working on his show--and, in fact, insists he's never been to Spain. Bobbi is presented as a nutcase for a couple of episodes, but it turns out that she was right all along. Dan was on a trip to France, got drunk, he and his friends crossed into Spain for a night...and Bobbi was far less attractive and went by "Roberta." When he figures all this out, Dan's apology is sincere and quite moving.
 * That 70s Show in an episode where Eric's attractive female cousin convinces Eric that she's not really his cousin and pretends to be interested in him, then humiliates him when he makes advances on her.
 * Interesting variation in Two and A Half Men where it happens to Alan of all people. The girl was very attracted to him in high school, and gave him many not so subtle hints. Alan wasn't mean to her, in fact they were great friends, but his general obliviousness towards women kept him from making a move. Still, she was quite pissed.
 * Ugly Betty
 * Caroline in The City featured a subversion: Annie was seeking revenge on several people who made her life hell during high school, only to be prevented at the last minute (for instance, her sadistic gym coach became a nun). This culminated in her parading in front of the apparently happy and successful ex-boyfriend showing him "what he missed" and finally asking him "Well, like what you see?". His response? "I can't see anything, I'm blind."

Video Games

 * Somewhat used in Tokimeki Memorial.  wasn't ugly by any standards, but apparently she wasn't pretty enough in Junior High so guys kept dumping her.

Western Animation

 * Parodied on The Simpsons; when Moe's plastic surgery makes him attractive, he begins hunting down all the girls who rejected him in the past. One woman apologised and offered to make it up to him. Moe was very confused.
 * When Leela returns to her Orphanarium in Futurama she attempts to show off her, legitimately, better life to the kids who bullied her as a child but they still feel they're better off since she still just a cyclops.