The opening of the 2006 Emmys had host Conan O'Brien wandering on to the sets of various TV shows. At the end, he walked into an empty house, only to be confronted by Dateline's Chris Hansen and find himself in the middle of a To Catch A Predator segment (which was hugely popular at the time). At which point his declaration of "It's not what you think", and all other truthful explanations for his presence we're dismissed by Hansen, no doubt because they sounded exactly like what countless other perverts who have been caught have said.
Dick, guilt-ridden, finally decides he must tell Mary everything about himself to have an honest relationship. He tells her who he is, where he's from, why he's on Earth, and who sent him. Being that they are at a Sci-Fi convention at the time, she merely replies that she is, in fact, an alien sex queen.
Another 3rd Rock example: when Dick is facing an IRS audit he finally breaks down and confesses to being an alien, to which the tax guy simply mutters, "Sorry, I've heard that one before."
And a third example involving Dick, this time while he's sick with a cold and, believing that he's dying, confesses to Mary, who thinks he's just being delirious.
Another example by Dick: when Dick dreams for the first time, he thinks he's gone mad and goes to see a psychologist. He ends up confessing his status as an alien to the psychologist, who thinks this is evidence Dick is experiencing major delusions.
12 Monkeys: Referenced in Dr. Reilly, whose first name is Cassandra here-no one believed her story that Cole vanished right before her eyes.
Jack Bauer on 24 repeatedly takes the role of the Cassandra, which is frankly bizarre when you consider his extensive field experience and the fact that he's almost always right. Though, to be fair the high rate of turnover on the show means that about the only person alive at this point who knows him well enough to trust him is the one who does, Chloe O'Brien. A lot of the field agents will believe him as well, it's just the higher ups that never do. There's actually a saying for this: "If everybody did what Jack Bauer told them to do, the show would have to be called 12." In fact, this trope gets played with in the final episodes. When Jack goes off on his own after the Russian masterminds behind the murder of President Hassan, Chloe for once doesn't believe his claim that they're behind it and refuses to work with him, due to recent events including the death of a woman he'd just started to become lovers with, and she feels he's not thinking straight because of it. Unfortunately, in this case, Chloe's also right: Jack isn't thinking straight. He wants to kill them all in revenge rather than just exposing them (well, he does kind of, but rather as just a back-up plan in case he dies in action as some type of final "screw you"). Did I mention said masterminds are also members of the Russian Government and killing them would cause an international crisis?
In The 4400, Maia had a vision that Jordan Collier was going to be assassinated. Her mother brought it to Collier, but he didn't believe her. Later seasons had a better track record of believing the psychic kid, eventually fully subverted when one of her visions was finally believed, only to be revealed that she was lying about it so that Collier and the rest of the P+ would let her and their other captives go.
Wil in Season 2 of The Amazing Race. He never missed an opportunity to tell his ex-wife Tara that helping out fellow racers Chris & Alex was a bad idea, and they should be concentrating on the race instead of helping another team. Though he was ultimately proven right when Chris & Alex passed them up in the finale, he was portrayed as a villain because of this... that and he was a Jerkass.
American Odyssey has an inversion, when Harrison Walters' dad Randall, a well-established news reporter with connections to the Establishment, comes to believe his son about the "dead" Sergeant Odelle Ballard.
Arrested Development: Michael and George Michael learn An Aesop about being honest with each other after Michael misinterprets his son's behavior. With his father repeatedly telling him that he can share anything with him, George Michael blurts out that he is in love with his cousin Maeby. The mood becomes deathly silent, until Michael realises it was a "joke" on him.
In Arrow Season 2, no one believes Laurel that Sebastian Blood is a Villain with Good Publicity, several people aren't even willing to entertain the idea. In fact, their actions just tip him off that she's on to him. He then exposes her addiction problems destroying her credibility, and arranges for one of his followers to take the blame leaving her doubting herself and sending her into a downward spiral. Fortunately, much later, and after she's stopped drinking, she's able to get proof and is believed instantly.
G'Kar of Babylon 5 has been described as JMS's Cassandra; at various points, he predicts what will happen, but no one believes him, mostly because they don't want to. For example, he tries to warn other races that the Centauri, having conquered the Narn, will turn their attention to others... which they promptly do. It was later revealed that both Delenn and Kosh knew some of his rantings were true but couldn't act in case it showed their hand too early. G'Kar calling Delenn on it when he finally found out was awesome.
Chief: How did you figure that out? Boomer: I'm a Cylon. Chief: That's not funny!
They later built a Bizarro Episode around this trope, with Helo trying to unravel a conspiracy that's just so stupid and outlandish it can't be true. It turns out to really be true and everyone walks away with egg on their face (even Helo) because of how stupid they all acted during the event. And then Ron Moore said: "Let Us Never Speak of This Again."
An episode of Beyond Belief: Fact or Fiction features a story where a little boy insists that there's a monster in his closet and is tormented mercilessly by his older brother and a pack of bullies because of it. Finally, the boy dares the brother to go stand in the closet with the door closed. The brother does, and, following some terrified screaming, their Mom opens the closet door to find that the brother HAS DISAPPEARED. The really scary thing? The story was listed as "Fact".
In season 3 episode 1, Hank is helping Walt move out. He takes a bag full of money and when he asks Walt what is making the sport bag so heavy, Walt replies: "Half a million in cash." Hank thinks it's a joke and laughs.
In season four, Hank stumbles on some evidence that connects Gus Fring to the meth trade. However, no one else believe him, so he investigates on his own.
It is usually in the earlier seasons but Giles and her other friends often do not believe the hunches Buffy gets about upcoming evil events and who is responsible for them. The most notorious example may have been when she told her friend that her new college roommate Kathy was not human and held up a bag of her clipped toenails as proof. They actually trapped her in a net, tied her to a chair, and tried to warn Kathy. Turned out she was a demon from another dimension. Although it's later revealed Cathy was slowly stealing Buffy's soul, which was causing her to act in a very bizarre manner. The end of the episode even indicates that Buffy just can't stand having roommates.
When Buffy first became the Slayer, she tried to tell her parents, who responded by putting her in a mental institution. She eventually stopped talking about it, and got to go home. Or did she?
It is, however, averted when Oz starts to become a regular character. He sees Buffy slay a vampire, and after briefly summarising the whole vampire/demon situation, Willow says "I know this must be hard to believe", only for Oz to respond "Actually this explains a lot."
In a pretty direct Shout-Out to the original myth, one episode features a girl named - that's right - Cassie, who has visions of the future that no one believes. Buffy actively saves Cassie's life twice on the day she predicted she would die, but she ends up falling to the ground dead anyway.
In a non-demon related example (well, at least initially), in the second episode of Season Three the Scooby Gang decide to throw a party to welcome Buffy home after she ran away, and excitedly decide to make it a Wild Teen Party. Giles is the only one to express doubt, arguing that Buffy is likely to be overwhelmed and that something smaller and more intimate would both help her acclimate and help everyone come to terms with the issues between them. The Scoobies sneer at Giles as being out of touch and overrule him — but it quickly becomes clear that the main reason they're hosting a huge get-together is so that they can technically welcome her home while avoiding her (and thus their issues) as much as possible. Needless to say, Giles is proven right, Buffy is overwhelmed by the party and hurt by her friends avoiding her, and things quickly get ugly. And that's before the zombies show up.
In Carnivàle, when Libby finally tells her mom the truth about why she and Jonesy were gone all day, that is, that they were kidnapped by some men who tarred and feathered Jonesy and left them in the middle of nowhere, until Ben happened to show up and heal Jonesy, and that Jonesy and Ben went off to find Ben's dad, she doesn't believe a word of it, even though I can't think of any reason why anyone would possibly make that story up.
She told her mom the story (which was true) because her mom was relentlessly telling her that Jonesy had run off. To shut her mom up, Libby told her the truth.
Another example in season 1, episode 2, when Apollonia breaks away from her paralyzed state for a brief moment, getting out of the trailer by herself to go tell Ben that he's "the one", before fainting again in front of him. When Sofie and the other carnies realize Apollonia's absence, Jonesy outright punches Ben, and nobody (maybe except Ruthie) believes a word he says when he tries to explain the situation. Justified, as even most of the carnies don't know the extent of Apollonia's abilities.
Paige spent a significant amount of time in season 4 insisting that Phoebe's ex-demon boyfriend Cole was in fact still an evil demon. Everyone else insisted that she simply didn't like him, and no one believed her, until he successfully turned Phoebe to the dark side and became The Source Of All Evil, with Phoebe as his evil queen. Needless to say, Paige earns the right to "I told you so."
In Continuum, Kiera has to tell so many lies that it's obvious to Agent Gardiner that she isn't who she says she is. She eventually gets sick of the facade and tells him she's a time traveler, but he thinks she's messing with him.
Gardiner: I saw you survive the explosion and fireball, and then vanish. Cameron: Yeah that. I'm a stranded time traveler from 2077 using technology that hasn't even been invented yet. Gardiner: That's bullshit.
According to a CSI: Miami flashback episode, Horatio was this when he first joined the Miami PD, dealing with lethargic cops who wanted to put the wrong man behind bars because he was the right culprit according to their antiquated methods — but not according to Horatio's observation and desire to try new methods.
Dexter, in the episode "Shrink Wrap". Dexter was already planning to kill his therapist for pushing three of his patients to suicide, so it didn't really matter if the therapist believed him or not.
Dexter: I'm gonna tell you something that I've never told anyone before. Dr. Meridian: Okay. Dexter: I'm a serial killer. Oh God, that feels so amazing to say out loud! Dr. Meridian: Well, you must be letting go, because I've never heard you make a joke before.
"The Fires of Pompeii": Played for tears when Donna screams at the people of Pompeii to run to the hills, rather than the beach. No one listens.
"The Woman Who Fell to Earth" has it discussed: when rookie cop Yaz wants to tell her superiors about the alien loose in Sheffield, the Doctor points out that they wouldn't believe her even if she could accurately explain what it is she's reporting.
One episode of Family Matters had Carl becoming especially angry at Steve Urkel for some reason, and throwing him out of the house. Steve was trying to warn Carl that the lamp he was fixing had a dangerous short circuit in it, and that Carl shouldn't plug it in, but the sneering Carl just ignored him and told him to leave. As Steve heads for the door, Carl plugs in the lamp and is electrocuted, and would likely have died if Steve hadn't come back and given him CPR.
In a first-season episode, Carl is providing security for Buddy Goodrich, the star of a super-popular TV show. He soon discovers that Buddy, despite his wholesome image, is a total jerk who thinks his stardom means he can avoid any consequences for his actions. When Carl arrests Buddy for assaulting him after being told to move his car from a handicapped-only space, everyone tells him that the officer must be mistaken, as there's no way that Buddy could possibly do anything wrong (it doesn't help that Goodrich is forcing his assistant to cover for him). Eventually, though, Buddy's true colors are exposed, and Carl is vindicated.
No matter how hard he tried, Chris the Crafty Cockney in The Fast Show couldn't get people to believe that he was a geezer, he'd nick anything. People would still insist the he look after something for them, whereupon he'd nick it.
River really is a seer, but since she's also a paranoid schizophrenic, people generally don't listen to her until late in the series.
Inverted in the Big Damn Movie. While the Cassandra of myth went insane because nobody believed her, River, who started out insane, regains her sanity after the crew finally believe her.
Debatable example but Jayne's opinions are often disregarded because he is, quite simply, an asshat but he also seems to have a point more often than not (finishing the job for Niska, Tams being trouble, bringing grenades on the bank job).
This is half the plot of First Wave. Our heroes try to prevent and reveal the first stages of an alien invasion. No one but a small collection of conspiracy nuts believe them.
In an episode of Frasier Daphne tries to explain that her Greek friend Zena will soon be arriving on her (the friend's) mother's ship, but the other person thinks she's talking about Xena arriving on an alien mothership.
Beyond the Wall, the White Walkers are returning, but very little is being done about it. Even those concerned by the rumours, like Tyrion and Ned, get distracted by other matters and forget about them. The Walkers' return also drives the wildlings into full-scale war with the Night's Watch, distracting the ancient order founded to stand against them. Stannis Baratheon is the only southern Lord who marches to the Wall to provide assistance.
The Stark motto, "Winter is Coming", is meant to be this. While other houses are busy playing power games (as their mottoes reflect), the Starks' first concern must always be the brutal, years-long winter ahead. Unfortunately, the current generation has been swept up in the power games instead.
Drogo's bloodrider Qotho was very right about not trusting Mirri Maz Duur.
Catelyn's warnings "Never trust a Greyjoy," and " Walder Frey is a dangerous man to cross."
Subverted between Davos and Melisandre. Davos is certain Melisandre cannot be trusted and the viewer can sympathize, but as time goes on and her allegiance does not waver, Davos seems to realize she's not treacherous, just a Blue and Orange Morality zealot. He still vehemently opposes her morality, but no longer questions her loyalty.
In one of the series' most ironic scenes, Joffrey (who is normally very short-sighted) is concerned by the very real rumours of Daenerys and her dragons, but Lord Tywin shoots him down with the cold and implacable yet incorrect logic that no one has successfully hatched a dragon in over a hundred years.
Correctly predicting a Cavalry Betrayal, Jaime and Varys warned the Mad King not to open the gates to Lord Tywin but were ignored.
Varys' repeated warnings are often ignored or forgotten, particularly those concerning the threat Littlefinger poses and the danger Shae is in.
Sansa warns Jon that Ramsay will toy with him in the coming battle. Jon brushes it off his half-sister isn't a warrior, after all. He then falls for every one of Ramsay's misdirections.
Margaery urges everyone to flee the Great Sept of Baelor after Cersei fails to appear for her trial, reasoning that she is up to something. She's right, but the Faith Militant won't let anyone leave.
Osha keeps saying that the White Walkers are coming, and that Winterfell's army should go North, not South. She also correctly interprets the comet in the skies over Westeros as heralding the return of the Dragons.
In season 2 of The Gifted, Thunderbird is held hostage by the anti-mutant Purifiers. John explains that the Mutant Underground is trying to help humans while the secret Inner Circle are the ones carrying out more brutal terrorist attacks. Given he's up against anti-mutant bigots, it's little wonder his words aren't believed (member Tom openly scoffs at the idea of mutants caring about humans). Turner seems ready to believe John as it would explain the wildly divergent operations. However, when members of the Circle join the Underground in attacking the Purifier camp, Turner thinks John was just playing with him and thus dismisses the idea of this division and convinced all mutants are dangerous.
Kelly in Harper's Island sees John Wakefield, the Axe-CrazyBig Bad, alive. No one else does and they all believe that she's insane. Turns out he's alive and dangerous. It's likely she's one of his first victims because she knows the truth. Kelly does have a habit of insisting that You Have to Believe Me! though.
"Headshot", an episode of The Haunting Hour, does an interesting twist on this trope, in that the villain is the one who tells the truth but isn't believed. The plot sees a girl named Gracie approached by a photographer named—surprise, surprise—Cassandra, who offers to take her photo and enter her in a contest to be the new "face" of Teen Teen Magazine. Though Gracie seems nice at first, she soon begins to go to extreme lengths to win, including spiking another contestant's (who's also one of her best friends) smoothie with an allergen and framing another member of her friend group for cheating on a test so she can avoid the consequences. Gracie's friend Lexi is sure that Cassandra has hypnotized her by taking her photo (especially because it's extremely clear that Cassandra is literally a Devil in Plain Sight), and goes to the photographer demanding that she undo the spell. But Cassandra explains that she hasn't done anything but encourage Gracie's true colors to show—if she was truly a good person, she would have deleted the photo that serves as her link to Cassandra immediately. Lexi refuses to believe this and decides to delete the photo herself... but Cassandra was speaking the truth: Gracie was the one who had to make the choice to save her character and soul over fame and beauty, and because she didn't, she's been permanently transformed into a hideous monster.
In the first episode, Claire announces at the dinner table, "I walked through fire today, and I didn't get burned." However, her mother thinks she's just being metaphorical and profound. Although to be fair, her brother was fairly suspicious and her mother isn't the sharpest anymore since the Haitian has been repeatedly wiping her memory which has resulted in the equivalent of punching her brain.
Angela Petrelli explicitly references the Trope Namer when she talks about her ability in the episode "Into Asylum". She also states that trying to work around this skepticism is what turned her into the Manipulative Bitch that we all know and love today.
Nobody listened to Patricia in her claims that Joy was in danger. Justified, because at the time she had been taking it too far and also erroneously believed Nina was involved. When she starts getting actual proof and cools down from her hatred of Nina, people start to actually listen.
When KT is framed for being a sinner who is working for Team Evil against Sibuna, no-one believes her when she states the truth, that she's innocent. Once Patricia reveals to her that she is really the Sinner, it just gets worse as the team would rather believe Patricia. This one is more idiotic on Sibuna's side, as Patricia was acting really out-of-character, which should have been a sign of something being wrong.
The focus of "iTwins". Freddie doesn't believe that Sam really has a twin sister after being pranked twice and not having seen Sam and Melanie at the same time. Also, Carly doesn't believe Spencer's accusations of the multitudes of abuse he got from Chuck.
In "iSpace Out", a little girl finds her way into the apartment, but when Spencer tried to show her to a police officer, she'd hidden somewhere.
I Didn't Do It: In the episode "Dance Fever", nobody believes Lindy as she repeatedly insists that Sherri is out to get her. They dismiss her frantic claims with the assumption that her (physical) fever is making her delirious, as Sherri is supposedly the nicest girl in school. But Lindy has already become wise to hertruecolors.
During an episode of The Invisible Man, Darien's ex-mentor shows up at his place to catch up and offer him a stake in her heist.
Liz: You went up for life on a third strike rap, it was in all the papers. How'd you get out? Darien: Well... if you must know, I was pardoned by a secret intelligence agency who surgically implanted a gland into my brain to turn me into a super-agent. Liz: ... You don't wanna talk about it. That's cool.
Yuri Aso in Kamen Rider Kiva is always being told Cassandra Truths ("I'm 105 years old.", "Your love interest is actually a homicidal monster who wants to use you to repopulate his race.") but never believes them. You would think a professional monster hunter would be less skeptical. Within the series, it's assumed that any monstery things other than the Fangire are extinct because of the Fangire. Why would a professional vampire hunter think that one of her suitors is a werewolf seeking to use her as a baby factory, that his friends the little shoe shine boy and the tall silent masseuse are a fishman and Frankenstein's Monster respectively?
In Kamen Rider Decade, self-proclaimed prophet Narutaki spends most of the series claiming that the title character is the Destroyer of Worlds and must be stopped. It looks like this trope because Tsukasa/Decade is doing his best to save parallel worlds and is actually connecting them. This isn't helped by the fact that even people like Kazuma Kenzaki say the same thing. It turns out Narutaki was right and wrong — Decade is supposed to destroy the Rider multiverse, but by connecting the worlds beforehand Tsukasa not only created a way to bring them back, but ensured that they would continue to exist forever.
What really makes this a Mind Screw: It's never clear what Narutaki's motive even is. In a couple of movies he's taken on the identities of past Kamen Rider villains, implying that he's Decade's Evil Counterpart and wants the Riders out of the way so he can take over, but in others he helps out the Riders (and the Rangers) seemingly without any kind of ulterior motive. Reportedly the creators of Decadedo have a full backstory set up for him that would explain everything, but going on ten years since the show ended it seems unlikely that the story will ever be told.
Lampshaded in Kamen Rider Gaim, where Kaito Kumon is fully aware that Mitsuzane "Micchy" Kureshima is a traitor who wants to kill Kouta Kazuraba, but doesn't bother telling him because he knows this trope will be in play. Kaito even makes a demonstration of it, suggesting that there's Mole in their midst while the traitor is in the room but Kouta brushes it off, and Kaito tells the traitor (who's shooting him a Death Glare) "See? He wouldn't believe me even if I told him." Later on, Yoko Minatojoins the good guys and tells Kouta to his face "Mitsuzane Kureshima has been deceiving you all along". Again, he doesn't believe it — and of course, this is the episode where Mitsuzane finally abandons all pretense and just attacks Kouta openly.
On Kyle XY, in one episode, Lori, Amanda, Hillary, and Declan go to a college bar to try and get a DJ for the Prom, and Declan starts a brawl. Later in the episode, when her parents ask her what she did that day, she tells them the truth, only to have it laughed off. Josh also says something along the lines of, "If you wanted it to be believable, you should've left Amanda out of it."
Many an episode of Leverage ends with the bad guy babbling hysterically to the authorities about how they've been set up, throwing around wild (and of course absolutely true) accusations about how the protagonists are not what they seem and have been conspiring against him. Framing the Guilty Party is optional but often involved, such as in "The Bank Shot Job" where they successfully frame a corrupt small-town judge for a bank robbery and have him dragged out of the bank in handcuffs screaming about how the whole thing is a conspiracy to destroy him.
In The Listener, Toby has a terrible time getting anyone to listen to the information he learns, mainly because he can't source it without revealing his secret. He had to put up with it so much in the first season that when he met Michelle in the second, he just came out and told her, using his powers to read her every thought and answer her verbally until Michelle realized he was for real.
In an episode of Lois & Clark, the recurring villain Tempus (a bored time-traveler from the idyllic future) travels to a parallel world, followed by the titular characters. After beating him, he has a press conference where he publicly outs Superman's identity... Cue Lois and Clark appearing in the crowd with Superman (from this 'verse) standing near Tempus. Naturally, no one notices the uncanny resemblance, as Tempus is taken to an insane asylum.
In "Raised by Another", Claire tells everyone that someone is attacking her in her sleep and trying to inject something into her pregnant belly. They all dismiss this as vivid dreams of a pregnant woman, only to feel suitably guilty when Ethan (who has indeed been giving her injections) kidnaps Claire and almost kills Charlie.
Another episode had Sayid traveling back in time to the 1970's along with some other characters. He is forcefed a pill forcing him to tell the truth. He does, and everyone thinks there must be something wrong with the pill.
Yet another episode had Hurley finally coming clean to Charlie that he was a multi-millionaire lottery winner. Charlie thinks Hurley's just messing with him.
On Lucifer, Lucifer is totally up front on how he's the Devil come to Earth, and annoyed people like cop Chloe don't believe him.
Chloe: I'm going to figure out your secret. Lucifer: It's not a secret when I'm telling you!
Therapist Linda spends the first season and a half assuming Lucifer's constant talk on God, Heaven, Hell and angels is all some grand metaphor for his life. It takes seeing Lucifer with his demonic face for Linda to finally accept the truth.
Played with in season three when Lucifer discovers Pierce is Cain, the world's first murderer, condemned to walk the Earth ever since. When Lucifer threatens to tell everyone who Pierce is, Pierce scoffs to go right ahead as "no one even believes you're the Devil."
Sure enough, when Lucifer tells Chloe, she doesn't believe him (Lucifer being wild after staying awake for a week doesn't help). In the season finale, Lucifer relates how Pierce told him that he was really helping the Sinnerman serial killer. When a shocked Chloe asks whey Lucifer didn't tell her this, he matter-of-factly states he did tell her clear off and she didn't believe him.
In season 4, Lucifer point blank lays it all out to Ella on how he's the Devil and everything and she thinks it's some sort of "character role play."
Later in the season, Chloe presses Lucifer to bring Ella and Dan into the loop. Lucifer basically points out that he's been completely honest to them on who he is and that they don't believe him. It would take seeing his "demon face" to buy it and, as he knows from experience, normal humans don't handle that well.
MA Dtv, in a parody of Medium. Allison repeatedly tries to warn the District Attorney of an impending homicide.
Allison: A man named Martin Grier is going to kill a woman named Susan Monroe at 466 South 27th street. DA: I don't understand... Who is Susan Monroe? Allison: She came to me in a dream last night. DA: I don't understand... a dream? Allison: ... Yeah... a dream. Every week I have a dream that helps you solve a crime... Every week. DA: I don't understand. Allison, just because you had a dream doesn't mean I can send my men on a wild goose chase. Allison: ... Well, you could send them to 466 South 27th street...
Malcolm in the Middle episode "Tutoring Reese" has problem child Reese complaining that his teacher, Mr. Woodward, is sabotaging his grades and is out to get him, but Lois counters that he's been using that excuse since kindergarten. When the much smarter Malcolm agrees to test his theory by doing a homework assignment for him, Reese gets an F again and Malcolm (and Lois when she finds out) confirm that Mr. Woodward really is out to get him.
This Trope was lampooned in an episode of Married... with Children, where aliens were coming into the house and stealing Al's socks. His attempts to tell anyone was met by both disbelief and sarcasm. (Marcie quipped, "Why do UFOs always visit idiots?") Eventually Al gave up trying to convince them and started trying to photograph them, hoping to sell the photos.
Done hilariously when he runs into the throne room, claiming he is a wizard, to protect Gwen who is about to be executed. Arthur (who doesn't know about Merlin being a wizard) comes to his rescue, by saying he is in love with Gwen and claiming that there's no way an idiot like Merlin could possibly be a wizard.
Merlin does this on an amusingly regular basis. When he's under Morgana's influence and Leon asks him why he needs a crossbow, Merlin cheerfully admits that he's going to kill Arthur. Leon laughs it off.
Merlin also regularly stumbles upon a Cassandra Truth. He either has no evidence or the evidence disappears.
This happened nearly every week in the early episodes of Monk; the implausibility of Monk's theory about the crime would be met with disbelief by Captain Stottlemeyer, and usually everyone else. The show wisely eventually dropped this, with Stottlemeyer beginning to accept Monk's explanations because he always turns out to be right, even (reluctantly) defending Monk against each week's stand-in skeptic.
No Tomorrow: Almost everyone thinks Xavier's a bit eccentric at least for thinking there's an asteroid headed to Earth, the professional astronomers included. Then his theory turns out to be right.
On The Office (US), Jim hid Andy's cell phone in the ceiling and started calling it so Andy would hear his phone ringing but have no idea where it was. Eventually, Jim said "Maybe it's in the ceiling," to which Andy replied, "Maybe you're in the ceiling!" and continued looking.
Once Upon a Time: Sure, kiddo. Everyone in this small town is a fairy-tale character with amnesia. Your (adopted) mom hauled you into a shrink and you end up calling the shrink Jiminy Cricket. You're a little messed up in the head... hey, wait. What's your mom doing with those shards of glass and that door in the graveyard...? Justified in that most of the town is cursed not to notice anything strange and of the three characters who aren't, two are already in on the secret and the third is a hardcore skeptic who lacks Henry's experience with the town's weirdness.
Subverted in "Living Hell". A guy is caught after he warned the cops about the actions of a Serial Killer who he's been telepathically linked to for the last several weeks. The cops initially believe that he's the killer, but after he provides proof of the neural device implanted in his brain, they believe him.
In "Ripper", Dr. Jack York's efforts to convince his fiancée Lady Ellen and Inspector Harold Langford that Jack the Ripper is an alien creature that jumps from body to body fall on deaf ears. They instead believe that he is the Ripper and he is arrested and committed to an asylum. Lady Ellen visits him in the asylum and tells him that she is going to America as she needs time away from England. Immediately afterwards, Inspector Langford tells him that he is retiring from the police force and that he will be Lady Ellen's escort in America, which he describes as "the land of opportunity." He then coughs up green bile, indicating that he has been possessed by the creature. As he leaves, Langford assures Jack that Lady Ellen will "hardly feel a thing."
Discussed in "Final Appeal". In 2076, the time traveler Dr. Theresa Givens has been sentenced to death for possessing and promoting the use of advanced technology. She compares herself to the prophetess Cassandra given that she has travelled further into the future and has seen that humanity will be wiped out by a devastating plague in 2105. The scientists of that time will be unable to combat it due to the anti-technology laws.
In "A New Life", Daniel follows Father, the leader of the religious community where he has lived for the last two years, into the woods and sees him transform into an alien and vanish in a flash of light. Later that night, Daniel, his wife Beth and their newborn son William leave the village but, after walking about ten miles, they are confronted by Some Kind of Force Field which blocks their path. They find Jacob, a former member of the community who left more than a year earlier, living rough in the forest. After bringing them to his cave, Jacob tells Daniel and Beth that the barrier covers an area of 20 square miles and even extends into the sky. The next morning, Daniel awakens to find that Jacob has been killed and Beth has been attacked. Before he can even process this, he is found by Father and other community members and brought back to the village. His efforts to persuade the others that Father is an alien who has framed him are unsuccessful and he is sentenced to burn at the stake. Beth was brain damaged in the attack and therefore cannot corroborate his story.
In "Promised Land", Rebecca warns the other humans that plants and fruits were poisoned by the Tsal-Khan, but many eat them anyway. Most of them die as a result.
Painkiller Jane: The neuro in Episode 4 ("Catch Me If You Can") sees the future, but like Cassandra no one ever believed him when he tried to warn people about coming disasters.
Root's Start of Darkness came when she witnessed her best friend get kidnapped (and was later murdered and secretly buried) and nobody believed her when she reported what she saw to the police.
The superintendent of an apartment building keeps telling everyone how he used to own night clubs in Miami and had a mansion where he kept a pet tiger. Everyone thinks he is just a harmless old coot but it is all true and the man sacrificed all his wealth to testify against the Mob. He is also planning to kill a stalker who is targeting a young women living in the building.
Played for Laughs when Finch infiltrates an insane asylum used by Samaritan as a front for their operations. He manages to get himself admitted as a paranoid schizophrenic by claiming that multiple gangs, as well as an all-seeing artificial intelligence, are out to get him, and that he goes by many aliases, all inspired by the names of birds, as a way to evade them. He clinches it by saying that is isn't paranoia if they're really after you.
Finch tried the same tactic to get himself disqualified from jury duty. However, he had to backtrack when he realized the Machine wanted him on the jury to keep track of a number.
A radio show host discovers that what people assume is just incidental static emitted by various electronic gadgets, is actually a bunch of coded transmissions. Finch and Root quickly realize that the man has stumbled on Samaritan's secret communication network. The radio host tries to expose the truth on his show but his show caters to conspiracy theorists so he has no credibility with the general public. Even the conspiracy theorists do not believe him.
In the Power Rangers Lightspeed Rescue episodes where the current Rangers team up with the previous group, the plot starts with a young girl discovering that aliens are secretly kidnapping people in the building her father works at. When trying to tell the genial old secretary on the first floor about it, the woman kindly informs her that monsters don't exist. Despite the fact that they live in an area currently infested by demons that attack weekly in an effort to completely wipe their city off the map so they can recreate their ancient society.
Power Rangers Zeo had a recursive Cassandra Truth, one time when Those Two Guys Bulk and Skull got caught up in an adventure and helped rescue some aliens. None of their friends believed them when they told the story - except the ones that were secretly the Rangers, but when they said so Bulk and Skull assumed they were just humoring them.
In Power Rangers Mystic Force Chip explained to the rest of the Rangers Vida had been turned into a vampire. They immediately write this off as impossible. Despite the Rangers being magicians and fighting monsters every week.
Proven Innocent: Bellows tells Maddie Adele really was guilty, saying they had DNA evidence against her that got suppressed on a technicality. She doesn't believe it, but he turns out to be right.
Olive: Why'd you fake your death? Is this an insurance scam? Are you and the pie maker in some kind of cahoots together? Chuck: I died. And he brought me back to life. Cahoots enough for you? Olive: If you don't want to tell me, just say so.
A Red Dwarf episode features a computer called Cassandra who can predict the future. However, her predictions aren't always as clear-cut as they seem and she tries to manipulate people by giving false predictions. In an interesting variant, though, she had been abandoned not because no one believed her predictions, but because no one wanted to hear them (because accurate truths about the future were often uncomfortable). She's not a perfect example of a Cassandra however, because her veracity was never really in doubt. However, Rimmer was so determined to believe that the 1st prediction of his death had some kind of Prophecy Twist that he actually managed to cause the twist and have another crewmember die whilst wearing Rimmer's name tagged jacket. He changed his tune, however, when the 2nd prediction of his death involved him having sex with Kochanski. Also, all her manipulations of the Red Dwarf crew came about because she was trying to alter her future and prevent her own death, which the end of the episode showed was impossible: she died due to Lister accidentally causing a chain reaction started by a piece of chewing gum.
In the late third season of Revenge, Victoria begins to suspect, and ultimately confirms, that Emily is Amanda Clarke. However, Emily sets things up so that Victoria's knowledge comes across as madness.
Lately, Hank of Royal Pains. No one believes that his father, Eddie R., is as toxic as he claims. Not even Evan. Especially not Evan.
In an episode of Seinfeld, George is truthfully attempting to explain away a very strange series of coincidences to an old childhood friend, who is convinced that George is going insane and doesn't believe his (admittedly odd-sounding) explanations.
Done in Sister, Sister: Tamera promises her dad she'll tell the truth for the day, then sees his girlfriend with another man at a movie theater. When she tells him, he's so disappointed that she broke her promise... until he catches the girlfriend with the other guy when they go out to dinner.
Played heartbreakingly straight in Skins, where Emily's coming out to her parents is assumed to be a sarcastic confession but is actually dead serious.
Emily: I've been making love to a girl... Her name's Naomi. She's rather beautiful. So I was nailing her. (beat) Rob: OK, OK, I get it. Nice one, had me going there! (continues to bust up laughing)
Government agents capture Lex who has trashed them effortlessly when possessed by Zod. He admits that an alien warlord has inhibited him and granted him with godlike powers, but naturally they don't believe him.
Actually, whenever someone talks about "little green men" from space. Pete "reveals" Clark's secret twice, once to Chloe and the second time to a random crowd, this way.
An episode of Space: Above and Beyond has Nathan as apparently the sole survivor of the 58th Squadron. His attempts to convince his superiors that the rest of his squadron is still alive are treated as symptoms of PTSD until almost the end of the episode.
Throughout "Point of No Return", Martin Lloyd (who's portrayed as a strawman believer in conspiracy theories) tries to convince O'Neill that he's an alien with suppressed memory, and succeeds only after they discover the escape pod in which he landed. In "Wormhole X-Treme!", the situation is reversed: now it's O'Neill trying to convince Martin that he's an alien and subconsciously based the Show Within a Show on the real Stargate program. Martin thinks it's a practical joke and writes down O'Neill's explanations as plot ideas for his show.
However, most of the time, the show doesn't succumb to this trope. It's not unusual for a character to experience something highly unusual, like seeing the future, and have everyone believe them. When you've seen as much weird stuff as the SGC has, you become more willing to believe in the unusual. In fact, on several occasions, one character has told the others about a crazy theory/experience... and is met with short-lived skepticism. Short-lived, because it becomes an excuse to list off the other crazy things they've been through. Averting this makes sense considering the SGC is a military organization; being a little paranoid is right in its wheelhouse, and they figure it's better to investigate a crazy possibility than dismiss it out of hand and potentially have it bite them in the ass later.
Although one of the earlier instances of this was in "There But For the Grace of God", when Daniel Jackson goes into an alternate universe and sees the Goa'uld attack Earth. He claims that he has the coordinates of where the Goa'uld will attack from, but no one takes him seriously until later on.
Samantha Carter: Daniel, it's not that we don't believe you. Daniel Jackson: So you do? Jack O'Neill: No. It's just that... we don't believe you.
Subverted later, after enough weird experiences result in people being more accepting. This usually results in whichever general is in command immediately believing the person without checking first, such as when Jonas claims to be seeing strange insect-like creatures throughout the base, which no one else can see. General Hammond orders an immediate lockdown.
General Hammond: The things I've heard sitting in this chair.
The Xindi story arc features a few of these. Most notably, Daniels has to bring Archer to the future to see the truth of the Xindi's misguided attempts to destroy humanity. When Archer tried to explain to the Xindi that not only was humanity not going to attack them, they were actually going to join forces at some point in the future and defeat a common foe, the Xindi council almost has him executed for his "blasphemy". Admittedly the Xindi did see this foe as gods/angels at the time and they had a different story of future events.
In the follow-up episode to the events of Star Trek: First Contact, Archer remembers that Zefram Cochrane related a simplified version of the events of the film. At the time, people had dismissed it as one of his drunken flights of fancy. He gives it more attention now that he's actually dealing with the Borg.
Subverted in the episode "In The Cards". Jake and Nog's Chain of Deals has attracted the attention of the bad guys, who refuse to believe they're going to all that effort to get a baseball card for Jake's dad. Jake therefore decides to take Refuge in Audacity, and instead claims they need the card for a secret Starfleet mission; Willie Mays is a time traveller, and they have to find out what he was doing in the past. After a moment of uncertainty Weyoun says "I believe you. That is, I believe your first story."
In the same episode, Jake and Nog's attempts to tell station personnel that something weird is going on (without mentioning the card) consistently fail when they get to Dr Elias Giger and his "cellular regeneration and entertainment device" (understandably, since the implication of the episode was that this was Technobabble by Star Trek standards). Weyoun turns out to believe that as well.
Stranger Things: Even when Will's "body" is found in the ravine, Joyce insists that her son is not dead, and that he communicates to her through the lights in her house. Understandably, everyone else in town just thinks she's going mad with denial and grief, especially as she has a history with mental instability. Except she's right.
Played for laughs in Supergirl. A waitress asks Kara how she can eat sticky buns and stay so thin. Kara replies, "I'm an alien," with a completely straight face, and the waitress laughs and walks off.
Gordon Walker in finds out about how children like Sam are supposed to be part of a demon army and tries repeatedly to convince Dean that Sam is evil and must be killed. And then in the beginning of Season 5 — oops! — it's revealed that Sam is, and has always been destined to be, the vessel for Lucifer. But then in the season 5 finale it's subverted, because, as Bobby had pointed out in the previous episode, if anyone could overcome the effects of Satanic possession, Sam can. Which, thanks in part to both The Power of (Brotherly) Love and to Dean's heroic stubbornness/suicidal co-dependency, Sam does.
Sam and Dean invoke this in "Sam, Interrupted" when they need to con their way into a mental institution for a hunt: They tell the psychiatrist in charge all about their Demon Slaying, knowing that no lie they can make up will ever sound crazier than that.
Taken: Due to the aliens' psychic powers, people in close proximity to them suffer serious health problems and often die as a result. The first sign is typically a Psychic Nosebleed. In "Beyond the Sky", Sally Clarke develops one after spending only a few minutes in John's presence. It eventually becomes apparent that the aliens are so interested in the Keys family, continually abducting Russell, Jesse and Charlie over the course of almost 50 years, because they are immune to the harmful effects that typically come with prolonged exposure. In "Beyond the Sky", Russell is the only one of the ten men aboard the B-17 bomber who were abducted on August 1, 1944 to survive more than three years after being exposed. The immunity of the Keys family is crucial to the aliens' attempts to create a viable hybrid.
In the 1986 Jim Henson TV special The Tale of the Bunny Picnic, Bean (Steve Whitmire) tries to convince his older brother Lugsy (the late Richard Hunt) that he saw a dog (Jim Henson himself) in the lettuce patch, but Lugsy keeps telling him that there is no dog. It isn't until the dog attacks the bunnies' community that Lugsy finally believes him...
In a Tracker episode, a fugitive kills someone inside the bar after Mel lets people in during a snowstorm. Cole tells Vic about his being an alien searching for alien fugitives, and Vic naturally thinks he's crazy.
One Tru Calling storyline involved a journalist investigating why Tru was present at so many crime scenes. In the end, an exasperated Tru told her about how she relived days... to which the journalist scoffed and promised to unearth the real truth.
An episode of the original Twilight Zone features a man who travels back in time, right before Abraham Lincoln is shot and killed at Ford's Theatre. He tries to prevent Lincoln's assassination and ends up in jail for acting "drunk" (until he is bailed out by John Wilkes Booth, who believes his story).
"The Time Element", the rarely-seen TZ pilot that aired on the Westinghouse Desilu Playhouse, has a similar plot about a man traveling back to 1941 Honolulu just prior to the Pearl Harbor attack.
The famous "Nightmare at 20,000 Feet" episode. A man on a plane sees a gremlin on the wing of a plane. He tries desperately to warn the crew. They don't listen.
In "Hocus-Pocus and Frisby" a guy known for telling tall tales about himself is abducted by aliens who believe his stories and think he's some sort of superhuman. He escapes by playing his harmonica which hurts them. He finally has a ridiculous story about himself to tell that's actually true which of course, nobody believes.
Used somewhat originally in the V miniseries; the aliens announce their presence and are apparently very upfront about why they are visiting Earth. It's revealed however, this is just a fallacy and the actual reason is much worse. However, they bring cures to diseases we still haven't found among other helpful things and most are reluctant to listen to any who speak out against them. Things get worse once they start recruiting young people as eyes and ears to report on any who might oppose them.
"Crazy Ponnie" is about Tori trying to convince the others that Ponnie, a girl she met while in the bathroom, is a real person (and later, that she is trying to sabotage her) but they all believe Tori is losing her mind. Tori was telling the truth.
Voyagers!: Jeffrey, anguished about all the loss of life that's going to occur, tries to warn people that the Titanic is going to hit an iceberg. History continues on course because he can't get anyone to believe him.
On Wizards of Waverly Place, at first, Alex didn't believe Justin when he said that their Aunt Megan was just like her, but after Megan says that she doesn't like hard work, Alex comments, "Oh my gosh, she is just like me". Subverted later in the episode, when Justin rescinds his statement and points out the difference between the two of them: Megan never learned how to apologize or even admit that she was wrong.
In Wolfblood, Shannon spends a large part of the first season trying to convince people that there's a monster on the moors. At one point, she becomes convinced that Maddy is the beast... which is of course true. Unfortunately for her, everyone else just thinks that Maddy is dating Rhydian and that's why they keep sneaking off.
On Wonderfalls, inanimate objects with faces talk to Jaye Tyler. When her best friend, Mahandra, asks Jaye what's wrong, Jaye tells her the truth, which prompts Mahandra to tell Jaye that Mahandra is there to listen to Jaye when Jaye wants to tell her what's actually wrong. She also makes some references to the objects when talking to Eric, but he seems to assume that she's just a very strange person using odd metaphors.
Cassandra Spender, with her stories of alien abduction and alien intentions that were deluded and then real.
Amy Cassandra, who believed herself to be an abductee, appeared to have been executed along with her husband by Mulder, since Mulder's gun had been used to shoot them and he had their blood on his shirt. The truth here was not (necessarily) Amy's abduction, but that she and her husband died in a murder/suicide triggered by a radical psychiatric therapy that Amy, Mulder, and a local cop were receiving.
For that matter, Mulder, with his stories of aliens and government conspiracies.
The Lone Gunmen, with their government conspiracies that both rivaled and fueled Mulder's, were occasionally proven to be at least close to the truth.