Third-Act Misunderstanding

"I'm torn, because on one hand, I want to share something important that happened to me while we were apart... But on the other hand, bardic tradition demands that I withhold it all so that at some later point, you can accidentally learn an incomplete version and jump to all the wrong conclusions--thus leading to entertaining dramatic conflict later in our relationship."

- Elan, The Order of the Stick

Alice is keeping a secret, perhaps even a hidden agenda, from Bob and his True Companions. She may benignly want to befriend or romance him, or less scrupulously steal something from him, or gain his trust as The Mole and betray him. Regardless, she's holding back key facts about her background which would make him doubt her honesty or outright hate her. But before long she genuinely develops feelings for him, and may find she's Becoming the Mask.

This being drama, her secret comes out in the third act and Bob and Co. reject her utterly. For extra pathos, it'll be at an important event like their wedding or after winning some award. Protests to the contrary are chalked up to "more of your lies!" Of course, this revelation comes just as Alice needs Bob to believe her. The Reveal can come in the form of a friend or enemy from their past life who doesn't want to let go or forgive, a randomly found memento or internet search.

To solve this, Alice will have to make a large, impressive and risky act of "contrition" to prove she's not the original liar anymore. Alternately, a third character (lets say Charlie) reveal to Bob that Alice really is a different person and likes him, and he runs to the airport to forgive her before she leaves.

Often, Alice could have avoided this situation if she'd thought things out and revealed her secret earlier when it wouldn't be as damaging. On the plus side, this is one narrative circumstance in which it is all but guaranteed that The Reveal Prompts Romance.

Romantic comedy, as a genre, abuses this trope. Most Chick Flicks need to have the characters break up without losing audience sympathy, so some sort of misunderstanding usually drives the third act.

Compare with Third-Act Stupidity and Second Act Breakup.

Anime and Manga

 * Almost exactly what happens in Dr Koto Shinryojo (Doctor Koto's Clinic), both in the manga and in the drama (although that's even more egregious in the drama) : the hero is at first rejected, but after spending countless hours helping the villagers, he eventually become appreciated. And then a grudge-holding man with a Dead Little Sister arrive and reveal that the eponymous Dr Koto made a mistake in the past. The villagers immediately turns against him, even tough he saved the life of almost everyone on the island. So yeah, he never was actually a jerk (although he think he was one when he made the aforementioned mistake), but there's definitely the "something from the past come back, friends abandon the character" part you're describing.

Film

 * The Adam Sandler remake of Mr. Deeds features Deeds learning that his love interest Pam, whom he's about to propose to, is actually a reporter named Babe Bennett who was helping a network smear his name. Babe initially wanted to help smear Deeds, but found he was so nice to her and generous with his money, she fell in love with him. She goes to try and explain... just as a report about the whole story is broadcast on TV. Not only does a heartbroken Deeds leave New York City, he signs away his $40 billion dollar fortune.
 * In the first Alvin and The Chipmunks movie, Dave writes a letter explaining why he wants them to leave, which he immediately throws out because he realizes he cares too much for them. Of course, a few weeks later they find the crumpled piece of paper (I think it fell behind the bed?) and assume he really does hate them. This despite their marked improvement in their relationship. You can probably guess what's spoilered here, but The moral of the story? Always destroy hatemail you don't intend to send.
 * In Avatar, the thing where The Mole is revealed at the point where he's already past Becoming the Mask, and he tries to explain to his teammates that he's on their side now, but they don't believe him. Seems to be different from Reformed but Rejected as the character has already switched sides, but is rejected in light of past events.
 * Legally Blonde, when Vivian sees Callahan hitting on Elle, and Vivian assumes she's using her looks to get ahead.
 * She's All That and 10 Things I Hate About You both use a variant where a bet is placed and after The Hero ends the bet or stops following it, the Love Interest only then finds out and rejects him.
 * The first Shrek movie has the eponymous ogre overhearing a conversation between Princess Fiona and Donkey about "who could ever love something so hideous." Shrek, however, creates his own misunderstanding after telling Fiona he heard everything, even though he hadn't.
 * Parodied in Not Another Teen Movie where the guy revealing the truth goes into almost excessive detail.
 * The third act of Hitch has Sara discover that Hitch is the Date Doctor she thought was responsible for breaking her friends heart, so she runs a scathing report that reveals his identity and destroys both his business and the burgeoning relationship of his latest client... all because of a misunderstanding of the independent actions of someone who Hitch refused to work with because he was a sleaze. Strangely zig-zagged when Sara goes to apologize to Hitch and ask for a second chance in the reversal of the usual roles. Hitch rebuffs her, but then for some reason ends up chasing her down and apologizing to her.
 * Two misunderstandings in Notting Hill: First Anna believing that William had betrayed her by going to the press and Second William overhearing comments Anna made while filming.
 * Madea Goes To Jail had this when the male main character's fiance is exposed (and left) on the altar after he finds out that she's been padding cases with other crimes to bolster her conviction rating.

Literature

 * Pick a Sarah Dessen novel. Any Sarah Dessen novel. This will happen.
 * Mara, Daughter of the Nile by Eloise Jarvis McGraw has one of these when Mara, originally hired as a spy (twice, once by each side), becomes sympathetic to (and has fallen in love with) one side, at which point she is promptly found out by that side and the critical information she bears them is treated as suspect.

Live Action TV

 * Series 1 of Gavin and Stacey, when the fact that, which has been kept a secret, is revealed when . Unusual in that we never see the two lovers settle the ensuing argument; the episode ends with them possibly breaking off their relationship, the next episode begins by simply stating they've made up.

Theatre

 * In Philoctetes, Neoptolemus is pursuaded by Odysseus to lie to Philoctetes in order to force him to come to Troy. Once he meets Philoctetes, he begins to feel genuine sympathy for him. As Neoptolemus begins to contemplate doing the right thing, Odysseus shows up, the lies are revealed and Philoctetes becomes very bitter and angry towards Neoptolemus. Neoptolemus does make it up to him in the end, even at the risk of The Trojan War ending in failure for the Greeks.

Web Comics

 * Lampshaded in The Order of the Stick by Elan (see page quote). Elan talks about her affair with Therkla to Haley in the Denouement (also lampshaded in that very same strip) of the 4th book, and mentions this. He finally tells everything.

Western Animation

 * Teen Titans had Terra experience this in the aptly named "Betrayal". Just as she's about to confess to Beast Boy, Slade shows up and reveals it instead, taunting and baiting him with the news that . She didn't take BB's rejection well.
 * Which leads to a moment of Fridge Brilliance when you think about the episode's name. Yes,, but just moments before finding out the truth, Beast Boy had promised that he would be Terra's friend no matter what. So it refers to his betrayal as well.
 * Towards the end of Mulan, Mulan's true gender is revealed. Fortunately, Shang's life debt to her saves her life; but when she tries to tell her friends that there are Huns still alive and heading for the city, they disbelieve her both on account of her deception and her gender.