A Brother's Price: Princess Trini warned her sisters not to marry Keifer Porter (who, because of the special kind of polygyny practiced in their culture, would become her husband, too), but was not listened to. He turned out to be evil, but even got away with beating and raping her, claiming that she "provoked him" when confronted by her elder sister, who liked his pretty face.
Nobody believes the kids the first time they explain what's going on.
At one point the Andalites are sending the majority of their reinforcements to the Anati system instead of Earth, believing the majority of the Yeerk fleet to be there. The kids have inside information that the Anati system situation is an ambush (the asteroid fields are rigged with automated turrets and mines). When they hear this, the Andalite command assumes the kids are lying in an effort to become a priority.
The Boy Who Cried Wolf: After lying so much the villagers don't believe the little boy anymore, even when there's an actual wolf chasing him.
Suggested in the Damnatio Memoriae series by Laura Marcelle Giebfried, where Enim and Jack realize that a number of local girls who ran away have actually been murdered by someone on the island. Of course, given that they're constantly up to trouble and Jack has a reputation for thinking up wild conspiracy theories, they know that no one will ever believe them. To make matters worse, after Enim thinks he finds solid proof that they're right, he's diagnosed with schizophrenia, and even the reader can't be sure whose version of events is true.
Liz is in denial over her death until someone besides Chloe (who is a necromancer and thus can see ghosts) confirms it.
Rae is the only kid who doesn't believe the facility they're staying at is dangerous and will even kill them under certain circumstances.
Most books in the Goosebumps series by R.L. Stine, in which the protagonists' supernatural claims are disbelieved by parents and authorities. This is turned around in "The Girl Who Cried Monster"; the girl's parents don't believe that her librarian is a monster, not because they don't think monsters exist, but because she has a habit of making up outlandish stories (in fact, it's her Establishing Character Moment) and because they're monsters, and they thought they'd eliminated all nearby competitors long ago.
In Anne S. Lindbergh's The People in Pineapple Place August's mother does not believe his stories of Pineapple Place, an alley only he can see, filled with families, all of whom only he can see. However, it turns out that his mother is a Reasonable Authority Figure, and comes to believe him once she sees evidence of August's story (a child she (and August) can see, but no one else can, able to get away with considerable mischief, and a security guard, apparently making a fool out of himself in front of a large crowd of people, none of whom (except August) can see the girl he (truthfully) claims to have caught roller-skating in a museum).
"You don't understand, Professor. Harry Potter's coming - he's got a dragon!" "Someone's trying to steal the Philosopher's Stone", "Sirius Black is innocent", "Voldemort has returned", "Draco is trying to KILL people"...
Sybill (great-granddaughter of famous Seer Cassandra) Trelawney is regarded as a fraud by her colleagues and some of her students, but the mindful reader will notice that almost everything she "predicts" does happen, just not in the way she says it will.
An interesting twist on the classic trope is that anytime Ron makes a snarky comment, half the time he's right. In Philosopher's Stone:
"I'll kill Fred, he was going on about wrestling a troll." And they're doing just that a chapter or two later.
In Chamber of Secrets, on who's the heir of Slytherin: "Maybe he murdered Myrtle, that would have done everyone a favor." And the previous victim of the Heir of Slytherin is indeed her.
Less obvious examples in Prisoner of Azkaban: "She's still acting like Scabbers has gone on vacation, or something." Surprise, surprise, Scabbers wasn't dead after all, just hanging out in Hagrid's hut.
"What would [Hermione's Boggart] have been for you? A piece of homework that only got nine out of ten?" Lo and behold, Hermione's boggart is McGonagall telling her she failed everything.
Cornelius Fudge becomes like this in Goblet of Fire, refusing to believe anybody who claims a threat of Voldemort's return may actually be true. A lot of this is due to an obsession that Dumbledore is trying to replace him as Minister for Magic. (Rita Skeeter's malicious slander doesn't help either.) This ignorance causes him to make several foolish mistakes and continues into the next book, where even a mass-breakout at Azkaban isn't enough to make him see reason, and eventually leads to him being dismissed from his position. His replacement, Rufus Scrimgeour, is more competent but has his own problems.
The Dresden Files has a condition called Cassandra's Tears, resulting in a person having somewhat reliable visions of the future, which no one believes. If someone does believe, the condition may be cured — but it's easily faked and a common confidence scam among the magical community. Which probably contributes to the fact that no one believes the predictions. More medically, genuine cases are also easy to mistake for garden-variety seizures, so people not in on The Masquerade, or people in on it but not suspecting the condition, could end up trying to medicate the wrong problem.
Cassandra herself shows up in one scene of the Everworld series, displaying just how thorough her curse is. Despite knowing all about her story, and having lived in a world where every myth and legend from all cultures throughout history coexist, the heroes still refuse to believe a word she says, including the statement that she is Cassandra, thanks to the curse's influence. Just to rub salt in the wound, they actually consider making a concerted effort to believe her, on the off chance she is Cassandra, before forgetting what she actually said and then deciding ignore it.
Part of its premise is a deliberate subversion of the trope with author Richard Adams wondering "What if the Cassandra character was believed?" So in this book, the Waif Prophet, Fiver, is taken seriously by his brother and a select few who escape a doomed warren. There is some doubt when they enter the seemingly idyllic Cowslip's warren that Fiver warns not to enter while the gang ignores him. However, when the place's horrific secret is revealed, the company then accepts Fiver's counsel without question, such as when the group encounters a electricity transmission tower and Fiver firmly tells them that it is of no danger to them. Not all the rabbits who joined Fiver in the first place necessarily believed him (although Hazel did); they were dissatisfied with their life in the warren and thought they'd have it better elsewhere. It's only after he is proven right, both about their home warren and about Cowslip, that that all really start believing him.
An El-ahrairah short story in the book relies on El-ahrairah constructing a Cassandra out of a suspected spy so that he would lose his credibility and be ordered to leave the warren.
Chorus: Why do you cry out thus, unless at some vision of horror? Cassandra: The house reeks of death and dripping blood. Chorus: 'Tis but the odor of the altar sacrifice. Cassandra: The stench is like a breath from the tomb.
James Thurber's The Catbird Seat is about a man who plots to get rid of an incredibly obnoxious woman who works at his office; she's driven away most of his colleagues and is about to talk his superior into cutting out the man's department. The man, a clean-living, sober type who wouldn't hurt a fly, visits her apartment one night, at which point he drinks whiskey, smokes a cigar and discusses his plan to kill his boss using very harsh language. The next day, the woman tries to warn their boss of the man's plan... and is fired when the boss thinks she's having a breakdown.
In James Thurber's short story "The Unicorn in the Garden", a Henpecked Husband finds a unicorn in the garden, but his wife doesn't believe him, telling him firmly that there's no such thing as unicorns, and calls for him to be taken away to a mental asylum. The tables are turned when the officials from the asylum arrive; when she tells them her husband saw a unicorn in the garden, he meekly says that there's no such thing as unicorns, leaving her looking like the unbalanced one.
Children's book Voyage of the Basset (the movie Voyage of the Unicorn is based on it) has a mythology-loving college professor whose daughter, Cassandra, is specifically named after this character. Cassandra somewhat lives up to her name when her warnings to her father about what not to do and trouble that could be caused are completely brushed aside, resulting in him getting pissed off and acting nasty to her. He later apologizes when she turns out to be right. (In the movie, he has a nicer personality and the "ignored warnings" thing is avoided.)
A short story started with a boy not being able to sleep because he had a fly in his ear. Then all four siblings got up and had a midnight party, playing make believe games so that when the toddler told the parents the next morning they assumed it was a dream, until he mentioned the fly in his brother's ear.
Coraline calls the police to tell them that her parents are missing — and she thinks they were taken by the creepy lady with buttons for eyes who lives in the Alternate Universe connected to her house. The police tell her to go back to bed, sweetie.
A major part of the plot in the Warrior Cats book Dark River: when a crisis on RiverClan territory forces them out of their camp, the other Clans all start preparing for invasion, since they believe that RiverClan will now try to steal territory. Hollypaw is seemingly the only cat on the lake that notices that all these fears are founded on nothing but paranoia, and that by preparing for a battle, everyone is making it that much more likely to happen. Naturally, nobody listens to her when she says they should try to help RiverClan with their problem, or at least get more information about it before jumping to conclusions, because she's just an apprentice and they are all "more experienced".
Night's Dawn trilogy by Peter F. Hamilton. Joshua Calvert always tells a different story over how his father damaged his spaceship The Lady MacBeth, usually involving some form of selfless heroics. Eventually his girlfriend asks for the real story, and is given one involving terrorists and mysterious alien technology. Naturally she doesn't believe a word of it, much to Calvert's annoyance, but a short story by Peter F. Hamilton in another book reveals that he is in fact telling the complete truth.
In Pseudonymous Bosch's The Name Of This Book Is Secret, and its sequels, the main character is named after the character from Greek legend and is often not believed by adults. A fairly detailed description of the original Cassandra character is given in the first book.
In the short story collection Far North & Other Dark Tales, by Sara Maitland, the mythological story of Cassandra is retold as being the result of a Apollo severing her corpus callosum as revenge for her withholding the sex she had promised in exchange for the gift of prophesy. She can see the future, but because of her brain damage cannot articulate clearly enough to be understood.
Fanny Price in Mansfield Park tries to warn Edmund that Henry Crawford is constantly flirting with his sister. Who is engaged to someone else. This does not end well.
Miss Bates in Emma is also usually right, but her Motor Mouth tendency causes people to tune her out.
Charlotte Collins in Pride and Prejudice is also frequently right. Elizabeth assumes she's just jealous.
Elizabeth herself, when she tries to warn her father that allowing her sister Lydia to go to Brighton with the regiment will end in disaster. It does. He magnanimously tells her that "I bear you no ill will for being justified in your advice to me."
In Stieg Larsson's The Millennium Trilogy, Lisbeth repeatedly told authorities as a child about her father's abuse of her mother, but no one did anything and she wound up being institutionalised after attempting to take matters into her own hands. There turned out to be a giant government conspiracy responsible for covering up her father's crimes, so this isn't entirely a straight example of the trope. It's also largely responsible for turning her into the person she is today.
Played for Laughs at the end of the Dale Brown novel Flight of the Old Dog, where Patrick McLanahan casually tells his mother that he had just come back from bombing Russia. Mrs. McLanahan doesn't believe him. Also used seriously in Plan of Attack, where no one outside of the Air Battle Force believes that a Russian attack is coming, as well as in Edge of Battle where no one believes just how dangerous Comandante Veracruz's plan really is.
In Jill Paton Walsh's A Presumption of Death, retired dentist Mrs. Spright is paranoid and senile so nobody pays attention when she claims that there are Nazi spies in Paggleham. It turns out that she's right.
In Glen Duncans I, Lucifer, the fallen angel Lucifer casually tells people exactly who he is, despite being in a mortal body, only to be seen as joking or eccentric. He even continues being himself when in talks to create a movie on his life story.
Jared in The Field Guide, the first installment of The Spiderwick Chronicles, tries to tell his family that faeries are causing all the mischief that he is being blamed for...but, since he's nine and has been acting mischievously as of late anyway, no one believes him.
In the Honor Harrington books, the few Solarians that recognise how far behind the times their Navy is are often casually dismissed as alarmists and defeatists.
In Flyte, only Beetle and Nicko trust Septimus when he tells them that Jenna has been kidnapped by Simon Heap.
Jenna tries in vain to warn Septimus of the upcoming Darke Domaine in Darke.
Kill Time or Die Trying: Brad spends his entire first day at university trying to figure out where to go. The only reliable directions he's given come from a pair of stoners, who he ignores.
The Changeover: Laura attempts to inform her muggle mother as to what her brother Jacko's condition truly is, to no avail. Subverted soon after with romantic lead Sorry, who doesn't believe her at first but then drops by and confirms Jacko's condition himself.
Subverted in "Police Operation" by H. Beam Piper. The Paratime Police are shown to actively take action to make true accounts of Paratime doings seem false. Flying saucers, their existence, and the relevant 'smothering out' technique are particularly discussed.
Dave Pelzer wrote A Child Called It about his experiences as the victim of the third-worst case of child abuse in Californian history. The evidence was clear across his body on a daily basis, up to broken bones (and worse!), and yet the school officials took several years to conclude it was bad enough to intervene. Worse because the couple of times he tried to tell, early on, they'd just call his mother, and send him back to her, and she'd abuse him even worse - so he stopped trying to tell anyone at all.
In the Trylle Trilogy, Wendy's mother stabbed her at the age of 6, claiming that she was a changeling switched at birth with her son. Wendy's family is horrified, and her mother is put into a mental hospital. Wendy eventually finds out that she is a changeling troll and her mother's real son is being raised by Wendy's biological mother Elora.
Galaxy of Fear: Eaten Alive, a ship called the Misanthrope crashes on an uncharted world that the natives call D'vouran. The twenty surviving crew find an abandoned lab that they hole up in, not trusting the natives, and find that one by one anyone who leaves disappears. The scientists got the ground to come to life and eat people other than the natives. The captain, Kevreb Bebo, is able to leave thanks to a trinket that keeps him safe; when other people start landing on D'vouran he frantically tries to warn them, but isn't believed. There's never any evidence; when he takes the last of the crew with him to help convince people, she disappears too. Gradually, a sympathetic Tash Arranda does decide to hear him out, because she too is not believed when she talks about the bad feeling she has about the place.
In City Of Glass, Jace's outright dislike and suspicion of Sebastian raises a lot of eyebrows despite the fact that he was right all along.
The Malloreon: In Demon Lord of Karanda Polgara pronounces a curse on a grolim:
Polgara: You are now invincible. No one can kill you—no man, no demon—not even you yourself. BUT no one will ever again believe a single word that you say. You will be faced with constant ridicule and derision all the days of your life and you will be driven out wherever you go, to wander the world as a rootless vagabond.
Grolim: Who are you, woman? And what power do you have to pronounce so terrible a curse? Polgara: I am Polgara. You may have heard of me. Sadi: Do you think it was wise to reveal your identity, my lady? Polgara: There's no danger, Sadi. He can shout my name from every rooftop, but no one will believe him.
Not strictly a Cassandra, in that the grolim doesn't have perfect vision, but he does know at least one true and potentially valuable thing that he'd probably like to share.
Lolita. After emptying a couple of pistol magazines into fellow paedophile Claire Quilty, Humbert discovers some Upper Class Twits have turned up for a dinner party at his house. He confesses to killing the man, but they just joke that someone should have done it a long time ago. It doesn't help that a dying Quilty then staggers into the room.
In fact, MAD kind of summed up how this Trope works in their satire of Gremlins:
Billy: Why won't you guys believe me?
Cop: Because the police never believe the hero until it's too late. Haven't you ever seen old '60's sci-fi movies like The Blob?
The Warbrunn-Knight report in World War Z. Features detailed information on the first zombie attacks and forming patterns, and nobody in a position to affect meaningful change even reads it, save for Israel.
Demon pox in The Infernal Devices is considered to be an urban legend, and no one believes the protagonist when he says that it's spreading.
Kim Stanley Robinson's Icehenge plays with this. It's unclear whether the first third of the book is narrative or excerpts from a document that may or may not be contradicted by the rest of the novel.
As noted under film above, Gandalf's suspicions that Sauron had indeed returned and was amassing new armies were dismissed by the White Council. However it's subverted in that Saruman does believe him, and actually knows for a fact Gandalf is correct, he's just delaying acting for as long as possible in hopes of finding the Ring for himself, and fears driving Sauron out of Dol Guldur too soon will will interfere with his search. He only finally agrees to take action to avoid tipping his hand to the rest of the Council, and because Sauron has grown in strength to the point that he has now begun to endanger Saruman's planning.
A double example in The Lord of the Rings: after Gollum's temporary Heel–Face Turn due to Frodo's kindness, Sam refuses to believe Frodo's insistence that Gollum has indeed earned their trust. Meanwhile, Frodo dismisses Sam's warnings that Gollum is still a danger to themselves and the Quest. Ironically, it was Sam's own distrust that delivered the final Heel–Face Door-Slam when Gollum was genuinely considering repenting and abandoning his plan to betray the Hobbits to Shelob.
In the Ascendance Trilogy, Sage repeatedly tells Mott and Connor that he is the prince. They interpret him as expressing willingness/desire to be the one chosen to impersonate the prince. Frequently employed in various situations throughout all three books, often coupled with Sarcastic Confession.
In DO NOT TAKE THE SHELLS, Harris's friends don't believe him when he tells them about the Eldritch Abomination living below the water, although they do promise not to go back to the beach that day.
In Midnight's Children, one of the titular children has the power of Time Travel. They warn the other children of impending doom, but nobody believes them, and they eventually leave the Midnight's Children Conference out of frustration.
In Charlotte's Web, a spiderweb appears near the pigpen of Mr. Zuckerman's barn with the words 'SOME PIG' written in it. Later, more spiderwebs appear with more words. While everyone is praising the miracle and believe that the pig must be extraordinary, Mrs. Zuckerman is the only character who thinks maybe the spider might be the extraordinary one. Her husband quickly shoots the idea down.
The book, the 1971 film adaptation, and the stage adaptation all maintain this plot hole. The only version to explain it is the 2006 film adaptation, where it's explained that Mr. Zuckerman looked for the spider that spun the web, but couldn't find one.
In The Maze Runner, Gally repeatedly tells everyone that Thomas is not to be trusted, and that he probably has something to do with them all being stuck in the maze. Obviously, considering that Gally’s proven himself to be a Jerkass with a mean streak, most of the Gladers ignore him. It’s later revealed that Thomas and Teresa were actually the ones in charge of building and designing the Maze project.
In the first book of The Raven Cycle Noah consistently tells the others he's a ghost. His first line in the series is literally "I've been dead for seven years," yet the others ignore him until they find his body in the woods. When this happens Noah's response is point out that he did tell them several times.
It's a Running Gag in the Eighth Doctor Adventures novel Alien Bodies that Colonel Kortez of UNISYC keeps insisting things are "not what they seem", leading to the other characters dismissing him as a paranoid loon. And he probably is a paranoid loon, but the fact is everything he says that about turns out not, in fact, to be what it seems.
In Worm, this tends to happen whenever Skitter and the Undersiders try to warn the heroes about a greater threat. Somewhat justified as circumstances lead the heroes to believe the Undersiders are just using disasters to gain power or are corrupting people in some way.
At the end of The Dire Saga, Dire tries to explain her actions to the heroes and they assume she's inventing a story to justify villainy. Although, frankly, it is a little implausible that a low-level street gang is being led by history's most scary villain, long thought dead bu revived by a vampire, and is colluding with a group of techno-mercenaries to help a group of artificial intelligences to use Y2K as a smoke screen as an opportunity to kill off an earlier group of AI.
In Angel in the Whirlwind: The Oncoming Storm, the Commonwealth's stated reason for annexing the Cadiz system is so that the Theocracy won't get it. They know however much the Cadizians hate the Commonwealth and want the Occupiers out of Our Country, the Theocrats will be much worse. After the Theocracy forces the Commonwealth Navy to cede the system, local resistance fighters essentially give a "Dang, you weren't kidding" reaction to some Commonwealth Space Marines who got stuck on-planet with them.
Edgedancer (a novella of The Stormlight Archive): Szeth keeps on telling Nale and his acolytes that the Desolation they've been trying to prevent is already here, but Nale is too keen on Believing Their Own Lies to listen to him, and his acolytes trust him more than they do Szeth.
Literature.Second Apocalypse: Sorweel hates the Anasûrimbor family because they invaded his kingdom and killed his father. The goddess Yatwer puts a glamor over his face so that the Anasûrimbor family cannot use their Dunyain abilities to see his thoughts through his expression. Moënghus, who has no Dunyain blood, tries to convince Serwa that of course he hates the family, but Serwa insists on trusting her perceptions.