Someone claims to be raped. However, the accusation turns out to be a lie. Depending on the scenario, it can be a matter of this fact already being known by the audience or a plot twist. In the case of the former, the conflict is about the credibility of the accused instead of whether or not rape ever happened. This is a very specific form of a Wounded Gazelle Gambit.
In order to qualify for this trope, the following is required:
There must be an accusation of rape. The alleged victim does not need to be the accuser.
The person accused must be innocent beyond a mere court ruling of "Not Guilty". This is because the "not guilty" ruling could also be played off as the criminal getting away with their actions.
There are also a few ways this can play out:
Mistaken Identity: The accused did not have sex with the alleged victim at the time claimed. They may have had sex with the alleged victim at other times. This is generally seen in fiction with "stranger rape", cases where the alleged victim was too impaired to correctly identify the attacker or cases where the victim was also murdered and is unable to provide an identification. This is also the easiest type to clear up.
Post-coital Regrets: The accuser decides (or has it decided for them) after the case that the sexual encounter was rape based on embarrassment, new information about the accused or social pressure.
Malicious Slander: The accused is simply accused of having committed rape for the purpose of harming them. For this, there need not have been any actual sex or alleged victim at all. Usually does not involve legal charges, unlike some of the others as it is "just" a rumor.
Occurs in Bitter Virgin, by the Clingy Jealous Girl. Unfortunately, the one she's telling it to (the girl she's trying to scare away) is an actual rape victim, with good reason to know how victims behave afterwards.
The Crush. A 14 year old girl named Adrian develops a crush on an adult man named Nick. When he declines her advances she tries to get him in trouble, including using semen from one of his condoms as evidence to falsely accuse him of raping her.
In The Wizard, one of the main characters, a young girl, falsely claimed that a man touched her breast as a distraction so that he would be tackled by nearby bystanders and they could escape. The kids had run away and were on their way to a video game tournament and the man in question was sent to take them back to their parents.
In Wild Things two teenage girls falsely accuse their high school guidance counselor of raping them in order to get revenge on him for various slights. Later on we find out that the guidance counselor was in on it; he sued the parents of one of the girls for a hefty sum after the false accusation and then the three of them split the money.
To Kill A Mockingbird. During the Great Depression, a black man in Alabama is falsely accused of raping a white woman. Even though his innocence is proved during the trial, he is convicted anyway by a racist jury. He is later killed while trying to escape prison.
Both played straight and thoroughly subverted in the book Factoring Humanity by Robert J. Sawyer. The main character is accused of molesting his daughter, and he is known to the reader (but not the other characters) to be innocent. However, a significant plotline involves him striking up a friendship with another teacher on the same university campus who was falsely accused of raping one of his students. They discuss how their allegations have affected them and the older peer comforts the main character. Near the end of the story, however, the other character admits that he actually did it.
Of Mice and Men: shortly before the action in the story, Lenny had caressed a woman's dress - not the woman, just the dress - and she ran to the sheriff's office claiming rape. Lenny & George barely escaped town with their lives.
A Passage To India: During a trip to the Marabar Caves, Adela accuses Aziz of attempting to assault her. Aziz's trial, and its run-up and aftermath, bring out all the racial tensions and prejudices between indigenous Indians and the British colonists who rule India.
In Atonement, Briony Tallis thinks her older sister Cecilia is being sexually harassed by their childhood friend Robbie Turner. (The sex they had was consensual.) When her cousin Lola gets assaulted at night and doesn't remember anything, Briony concludes that Robbie must be her attacker, even though he's innocent. Only Cecilia and Robbie's mother believe him. Briony's accusation and testimony drive the plot of the novel.
The last episode of the original Law & Order features several teachers—including the Villain of the Week—whose lives were ruined by false accusations of molestation from their students. This includes a guy who tried to stop a student from taking a leak in front of the class, and was accused of sexually assaulting the boy.
There was a Baywatch episode where one of the male lifeguards caught one of the female lifeguards using drugs and she accused him of rape when he threatened to report her.
In an episode of MASH, a formidable and high-ranking nurse accuses FrankBurns of attempting to rape her when she gets caught trying to seduce him.
In Dexter, Lila falsely accuses Batista of rape (in actuality, they had consensual sex before she took a dose of rohypnol and injured herself in order to frame him) in order to pressure Dexter into getting back together with her, implying that she will drop the charges if he does so.
In The Bible, Potiphar's wife tries to seduce Joseph but he refuses. So she tells her husband that Joseph has raped her and he's imprisoned. Similar tales appear in many other old myths and tales, usually when a powerful but married woman tries to seduce a hero but is rejected.
South Park, "Wacky Molestation Adventure." The boys, upset at their parents' being too strict, call the police on their parents saying they "molestered" them. The parents are taken away. Soon there are no adults at all in South Park and they descend into a Children of the Cornesque society.