Similar to Mistaken for Gay, this is when a character is wrongly accused of cheating on his spouse or girlfriend because of a series of misinterpreted clues. Usually these clues look very suspicious (a bra left in his apartment, lipstick on his collar, a piece of Affair Hair, a romantic note) and the real story is hard to believe. The wrongly accused may protest "Wait! I Can Explain!" or "It's Not What It Looks Like!" If the spouse or girlfriend is another major character, the conflict will be resolved; if the love interest is a Girl of the Week, she may storm off, thus ending the relationship.
Shows aimed at kids also have a version where it's the kids who wrongly suspect that one of their parents is cheating on the other. Other variants include the love interest discovering that the would-be boyfriend thief is actually a relative, a member of an Incompatible Orientation, or in rare cases, even the wrong gender.
One notable property of this trope is that it can be used on shows that, because of their genre or their scrutiny by Moral Guardians, could never use a plot that involved actual adultery. But you can get some of the same benefits from a story where a character wrongly believes that adultery is going on.
See also Mistaken for Index. Often part of an Idiot Plot; and can result in Poor Communication Kills, at least figuratively. May be resolved with The Grovel.
In the State Farm "State of Unrest" commercial a woman finds her husband on the phone with a (male) State Farm rep at 3am and assumes he's talking to The Other Woman. She grabs the phone and starts talking to him.
Wife: She sounds hideous. Husband: Well, she's a guy, so...
Anime & Manga
Forms the basis of some of the plots in Ah! My Goddess, when Belldandy suspects Keiichi is cheating on her. In the end it always turns out to be a Not What It Looks Like moment, or in Hild's cases, manipulating him and the goddesses in order to try to steal him away.
Happens a lot in Baka to Test to Shoukanjuu to the two main guys, Akihisa Yoshii and Yuuji Sakamoto. In one hilarious incident, Shouko demands access to Yuuji's cellphone, and after she starts to strip him in order to find it, he gives it to her. She then finds a text written to him by Akihisa, asking if Yuuji would let him stay over at his place that night. She immediately assumes that they're in some kind of illicit relationship. Later the main characters go to his house for a study session (and to see why he didn't want to go home), and the two girls who like Akihisa immediately also start questioning him as to why there's women-related clothing and items in his apartment (which turns out to be his sister Akira's stuff, who's visiting him to make sure he's not goofing off).
Himeji: This bra isn't your size, Yoshi-kun...
Surprisingly, Pokémon featured an instance of this in one episode — May's father Norman and the local Nurse Joy were actually setting up a fireworks display celebrating Norman and Caroline's anniversary.
Caught in bed with a girl snuggled up to him, both of them asleep (the girl slipped into his bed while he was asleep).
Caught with a girl attached to him like a barnacle while both of them are naked (she sneaked after him into the furo as a cat and glomped him).
Caught shoving a girl down onto a table while grabbing one of her breasts and shouting "Now give it up!" (No, seriously, it really isn'twhat it looks like.)
Ironically, the one time he did want to be Mistaken for Cheating in order to get out of a commitment, he used a very literal Lipstick Mark (which he applied himself in female form) and it was casually brushed off by the girl in question.
The entire student body always jumps to the conclusion that Ranma is cheating on Akane.
Girl #1: Don't just stand there, Akane.
Akane: What do you mean?
Girl #2: Ranma is making moves on another woman again.
Thanks to a misinterpreted conversation eavesdrop in a chapter of The Tyrant Falls in Love, Morinaga comes to believe that Souichi is having a sexual relationship with Isogai. (The truth is far more embarrassing for Souichi: he was blackmailed by Isogai, who discovered the relationship between him and Morinaga, into singing the Doraemon theme song on karaoke nights at Isogai's place.) One part that does not follow this trope is the discovery of hickeys on Souichi's neck which Morinaga takes as solid proof, forgetting that he made them himself several days ago.
One Planetes episode begins with Lavie seeing Fee talking to Director Dolf; as he's on a balcony a good distance away, he can't heard a word of it and assumes they have ulterior motives. (They're actually discussing Fee's possible promotion.) When he tells the rest of Debris Secton about this, they come to the en masse conclusion she's cheating on her husband. This lingers until Ai finally just asks Fee about it. Jump Cut to Lavie taking a roundhouse kick in the chest for having started the rumor.
It didn't help that, previous to their work at Technora, Fee and Dolf were coworkers at a small firm, and thus are still friendly to each other in private.
In the manga version of Shaman King, Anna mistakes Yoh's new Oracle Bell as a pager that a girl gave him. It doesn't help his case that he had a long black hair on his clothes from Silva. In the game Power of Spirits, this is taken further when Meril, a girl Yoh had never met until this point, walks in and hugs him.
Happens in Seto no Hanayome when a series of events results in Lunar's father walking in on Nagasumi rubbing Lunar's ass. Everyone immediately assumes that he was cheating on San, including San herself. Despite the fact that this exact circumstance (mermaid gets wet, reverts to mermaid form, forcing Nagasumi to dry them off, only for them to turn human at the worst possible time) has happened to her.
Kyou Kara Maou: Wolfram is always accusing Yuuri of cheating and Yuuri never is.
Lampshaded in YuYu Hakusho, when Keiko misunderstands Yusuke's relationship with Botan, who is just a friend and a partner to the spirit dectective.
In Bakuman。, Akito Takagi, seeking ideas on how to write female characters, meets fellow mangaka Yuriko "Ko Aoki" Aoki, but keeps the meetings secret from his girlfriend Kaya Miyoshi out of fear that she would get jealous over him turning elsewhere for advice. While there, he meets his former classmate and academic rival, Aiko Iwase, who gives him a copy of her book with a letter inside. Miyoshi then discovers the letter and becomes quite upset until the misunderstanding is cleared a few chapters later.
Shi from Wandering Son mistakes Yuki for cheating with one of the protagonists, Takatsuki, in his first appearance; she's touching him rather suggestively on the face. The creepy part is that Yuki is in her mid-twenties at youngest, and Takatsuki was only eleven.. And a girl who feels that he's actually more like a boy, though neither Shi nor Yuki knew that (though Yuki seemed to have guessed).
Used in one of the Kimagure Orange Road OAV's, Message in Rouge (And the manga story it was based on.) Madoka has an Heroic BSOD and runs away from home when she believes her beloved dad is cheating on her also beloved mom, staying in the Kasuga apartment when Kyousuke is home alone. It turns out Mr. Ayukawa was checking on the well-being of a co-worker who had a bad eye, and Mrs. Ayukawa knew it all the time.
Played much more for laughs in the OAV that introduced Akane. She mistakenly believes Kyousuke is cheating on Hikaru with Madoka, goes on to give his "other woman" a piece of her mind... and is left starstruck by her.
Happens between Jeremy and Vivi in the earlier chapters of A Cruel God Reigns. Jeremy is forced into having sex with Greg so that he will marry his mother Sandra, and eventually Vivi notices the subtle differences in Jeremy's reactions to her affection, his kissing, reactions to being touched, suspicious reactions to being asked, etc. Although Jeremy does actually cheat on Vivi, it falls into this troupe because it is against his will and it affects him very negatively, eventually ending in Jeremy killing Greg by Make It Look Like an Accident and Vehicular Sabotage. Jeremy tries both I Can Explain and Not What It Looks Like to try to keep Vivi as his girlfriend.
Also happens with Lilia. Greg suspected that she cheated on him while the two were in Boston and that Matt is an illegitimate child.
Mary Jane Watson's Aunt Anna once tried to get her to face the "truth" about Peter Parker's apparent infidelity, what with all his sneaking around at odd hours and missing commitments with the flimsiest of excuses; this aggravated MJ enough to evoke a Sarcastic Confession: her husband was really Spider-Man.
A story in Shock SuspenStories #11 called "Three's a Crowd" had a paranoid man believing his wife was cheating on him with his brother and he murders them both. It turns out they were being secretive because they were planning a Surprise Party where the husband would find out she was pregnant.
Another story in the same comic, "The Tryst," was based around a man being extremely possessive of his much younger Trophy Wife. When she mentions a new male friend, the husband believes she is having an affair, and shoots her friend dead from a distance. The friend turns out to be a young orphan boy she had been caring for because her husband refused to have a child with her.
Superman had to deal with this in one story when Lois discovered a pair of Lana Lang's panties in their bed which the jealous Lana had planted during her last visit.
Jimmy Olsen has been known to be Disguised in Drag on more than one occasion. After one such example, though, Jimmy got read the riot act by Lucy Lane. Turns out Lucy found in his apartment the purse, perfume and jewelry that were part of the masquerade lying around his apartment, but she thought he was dating someone else behind her back.
In one comic, The Joker decides to mess with a rehabilitative Two-Face by suggesting that his girlfriend and best friend Bruce Wayne are fooling around behind his back and want to keep him locked up in Arkham. To give it one final push, Joker has one of his goons make a fake newspaper article that Bruce and the girlfriend were getting engaged and slipped it to an already suspicious Two-Face. Two-Face then loses it, escapes from Arkham, proceeds to kidnap his girlfriend and attempt to murder Bruce.
A Pikachu in Love has an odd platonic, 'best friend' version. When Ash wakes up the morning after Pikachu spent the entire night up with Pichi, he's confused as to why his friend is still sleeping when he's usually up at this hour. Deciding not to disdurb him he takes Bayleef fishing with him instead. Unfortunately, Pikachu wakes up a few seconds before they leave, and wonders why Ash didn't take him instead. This is what gets Pikachu to really consider how strong of a bond with Ash he actually has...
In Through A Diamond Sky, this kicks off the plot. Jordan thinks that Kevin's secretive behavior, bizarre references, and long periods of no accountability are proof she's married a cheater - again. (He's husband #2). While it's true that he's up to a lot of very strange things in cyberspace, an affair isn't one of them.
In the Ben 10Hero High: Sphinx Academy, Ren suspects this of Ben when she has noticed him hanging out with Julie several times, though more often than not he is getting help from Julie to get a gift for Ren for their upcoming anniversary. It doesn't help that Susan, one of the Big Bad's lackeys and instructed to cause tension between the couple to slow them down, is the one encouraging the idea.
In The Many Secret Origins Of Scootaloo, Mrs. Cake catches Fluttershy giving Twilight Sparkle mouth-to-mouth, and immediately spreads the word that Twilight is cheating on Rainbow Dash. Twilight and Rainbow Dash aren't even dating.
The Reading Rainbowverse has a... complicated example, in that Big Mac thought he'd made it clear that he'd broken up with Fluttershy, but Fluttershy didn't get the memo, and the fact that Mac was interested in a relationship with a changeling of all things just made the situation more awkward. It ended up with Fluttershy going on a drunken rant and then getting suplexed off a dragon, after which she spent a couple weeks in the hospital high on morphine...
Films — Animated
The Incredibles: Helen finds one of Mirage's hairs on Bob's suit, then overhears the second half of their phone conversation before he suddenly leaves on a "business trip." It's never explicitly mentioned, but her thoughts are pretty clear. The outtakes, meanwhile, include scenes where she outright asked him if he was seeing another woman.
To make matters worse, when she does finally find Bob inside the villain's lair, it's right as he's hugging Mirage.
Films — Live-Action
Mabels Blunder (1914) involves Mabel getting jealous after seeing her fiance hugging an attractive woman. The woman turns out to be his sister.
In Enchanted, Giselle, just coming out of the shower, trips and falls on top of Robert, who had tried to catch her. Then Nancy walks in...
Subverted in the Brazilian movie Meu Tio Matou um Cara ('My Uncle Killed a Guy') Éder finds a series of pictures that seem to indicate that his girlfriend Soraya cheated on him with Kid. She then explains to him that he looked at the pictures in the wrong order, and they tell the story of a perfectly innocent afternoon. Later, the protagonist Duca mentions to a friend that the sun's position in the sky don't match Soraya's story, and Eder was right after all.
At the end of Pirates of the Caribbean 2: Dead Man's Chest, Elizabeth passionately kisses Jack, not knowing that her fiance Will is watching. She was actually distracting Jack so she could handcuff him to the ship.
Humorously inverted in The Naked Gun 33 1/3. Lt. Frank Drebin first tells his wife Jane he's seeing another woman as a cover story, then Jane misinterprets a clue that leads her to believe that Frank really is cheating on her, so she forgives him.
Frank Drebin: I swear, it's another woman!
In Knocked Up, Pete is making up lies about work and sneaking out of the house so that he can go see his fantasy baseball league.
In Transformers: Revenge of the Fallen, Mikaela walks in on Sam apparently sharing a passionate moment with an attractive co-ed, and storms off in a huff as Sam desperately protests that it's Not What It Looks Like. Said co-ed is actually a sexually aggressive Decepticon Pretender trying to get at the data embedded in Sam's head; the instant Mikaela leaves, the Pretender gives up on the Fake-Out Make-Out and starts attacking Sam properly.
Inverted in The Shop Around the Corner: Mr. Matuschek knows his wife is cheating with one of his employees and comes to the conclusion that it is Mr. Kralik. It's really Mr. Vadas.
Occurs in Who Framed Roger Rabbit, when Jessica Rabbit visits Eddie Valiant to ask for his help with her case. On the line "I'm desperate Mr. Valiant, can't you see how much I need you," Valiant's girlfriend walks in. It doesn't help that his pants are down as well.
That Lady in Ermine had this, due to Executive Meddling. Angelina's husband thinks she slept with the Colonel to make him leave (it was the ghost of her ancestor in a dream, convincing him to leave).
An odd example in Top Hat. Dale has confused Jerry for her friend's husband, Horace. Jerry loves Dale and Dale is convinced he's trying to have an affair with her.
In The Hand That Rocks the Cradle, the nanny gets the husband to plan a surprise party for his wife with their friend, who is the only one around the husband who smokes. When the wife finds her friend's lighter in her husband's suit jacket, she thinks he's having an affair, which she calls him out on with the next room full of party guests.
The main plot of Nothing To Lose is kicked off when the protagonist thinks his wife is having an affair with his boss and decides to rob the company in revenge. The evidence is seemingly incontrovertible (the boss' cufflinks are on the table, and they are going at it in the bedroom), but if he'd actually tried confronting them, he'd have found out they were relatives he wasn't told were visiting (and had a sudden case of the randiness). And the cufflinks had been there for months ever since the guy lost them during a party.
In Mystic Pizza, Daisy (a poor Portuguese Catholic girl) sees her love interest Charles (a wealthy WASP guy) talking to a very attractive woman at a party. Consumed with jealousy, she persuades a lobsterman to dump his entire day's catch into Charles's expensive convertible sports car. Daisy then discovers that the woman Charles was talking to was in fact his cousin, leading to a memorable exchange:
Daisy: I fucked up. Charles: Yeah... but you gave it a 100% effort!
In 24-hour Party People, Tony Wilson (Steve Coogan) is receiving a blow-job in the back of his van when his wife unexpectedly opens the rear doors; all he can do is scream, "It's not what it looks like!" Oh yes it is.
In the film Shall We Dance, a woman becomes suspicious of her husband when she realizes that he's displayed the same traits that a friend's cheating husband is—late nights, distraction, etc. As it turns out, while the husband is bored with his life and having a mid-life crisis, he isn't cheating on her, he's taking dance lessons. However, the wife is still concerned about the possibility of him having an affair with one of the dance instructors.
Used as Mind Screw in Internal Affairs (1990). Dirty Cop Dennis Peck knows the protagonist, Internal Affairs officer Raymond Avila, is following him, so he arranges to meet Avila's wife in a cafe where he discusses some innocuous business. He then assaults Avila in an elevator, taunting him about having sex with his wife and throwing a pair of panties in his face (which don't even belong to her).
In Finding Neverland, James's wife and several others believe that James and Sylvia are having an affair when he spends more hours with her and her sons than at home. In reality, he only saw her as a platonic friend.
Subverted in Reginald Hill's Dalziel and Pascoe novel Death's Jest Book, in which Sgt. Wield gets into a paternal relationship with a rent boy that sure makes it look like he's cheating on his partner—even Dalziel thinks so—except that he isn't. Ironically, Wield's partner is the one who hangs a lampshade on the situation by pointing out that he has complete faith in Wield's motives.
Turns up regularly in the works of P. G. Wodehouse, often on the most hair-triggered and flimsy of pretexts.
Happens with Denise and Spade in the Night Huntress books when she's told that "the vampire" is in a bedroom with another woman. Becomes a Crowning Moment of Funny when she furiously pounds on the door only to find she's got the wrong vampire.
Used tragically in Needful Things. Fiancees Lester Platt and Sally Ratcliffe are led to believe that each is cheating on the other. Lester beats her unwitting suspected lover half to death before being killed by the police, while Sally hangs herself in her closet.
In the novelization of Revenge of the Sith, Palpatine (who is fully aware of Padme's and Anakin's secret marriage) insinuates that Padme is cheating on Anakin with Obi-Wan. When Anakin visits Padme's residence to confirm these suspicions, he detects signs that Obi-Wan did visit her. While Padme is able to assuage his temper, Anakin never really lets go of the insidious notion. Later on Mustafar, when Anakin sees that Obi-Wan stowed away on Padme's ship, he takes it as a sign that Padme had betrayed him. Cue Force Choking.
Septimus Heap: When Sally Mullin sees Silas and Marcia fleeing from the Custodians she thinks they are eloping. Sally also thinks Jenna was conceived by Sarah in an affair because Jenna doesn't look like Silas or Sarah (Jenna is actually secretly adopted).
A retroactive variant comes up in the backstory of Paladin of Souls. The cover story of espionage and bribery used to explain Chancellor Arvol dy Lutiz's death in the dungeons of the royal castle some 16 years back was fairly transparent if one thought it through, so many (including the Chancellor's son) concluded that he had seduced the King's young wife.
The 16th Century Vietnamese Collection of Strange Tales has a story where a man after came home from the war got rejected by his 3 years-old son who claimed that 'his real father' has always 'appears every night, stands when mother stands, sits when mother sits'. The husband thought his wife was unfaithful with him and kicked her out of the house without waiting for an explanation. The wife, in despair, drown herself in a river. It wasn't until night came and the son pointed at his father's shadow on the wall, exclaiming 'That's my father!' that the husband realized that his deceased wife, who was just trying to calm their son (as he missed his father who was away for the war) with a white lie, had been innocent after all.
In one of the Stephanie Plum books, Steph thinks Morelli's reviving an old affair with Terry Gilman. Turns out it's a business matter, and she nearly screws up a case the FBI has been working on for months when she confronts him.
In Jinx High by Mercedes Lackey, Deke decides that the reason his father and Diana Tregarde are spending time alone together is that they're carrying on an affair. The real reason is to track down the mystical danger that wants Deke for lunch. To be fair, Deke wasn't aware of his own mystical abilities until late in the novel, and certainly wasn't aware that his parents had been part of Diana's Scooby Squad in their college days.
In a Nancy Drew mystery that crossed over with The Hardy Boys, Frank was half-carrying Nancy across a parking lot (she'd sprained her ankle), when they encountered her boyfriend Ned Nickerson. Already ticked off, he was even more so when she declined his offer of a date that evening, as she and Frank had to work on their case, thus confirming his incorrect suspicions that something was going on between them. The subversion here is that Frank and Nancy were quite attracted to each other throughout this Crossover series, so Ned's discomfort with their friendship wasn't entirely unjustified.
In a Sweet Valley High book, the twins and their older brother Steven become concerned about their father's apparently budding relationship with a coworker. It's never established if they were completely wrong or if there was something brewing before the father had the sense to end if before it could really get started. Earlier, in a Sweet Valley Twins book, the kids become convinced that their mother is having an affair with one of her clients and is planning to run off with him, even though there have actually been zero signs to that effect—they've simply taken everything nice that their mother says or does as romantic interest and when they take to spying on the man, they mistake his phone conversations with his girlfriend for lovey-dovey chats with their mother.
In the House Of Mirth, Lily Bart is unfortunately—and wrongly cast—in the role of the "other woman. Once by a so-called friend who is trying to cover up her infidelity, another time by a man whose advances she has rebuffed''.
The Teenage Worrier's Guide to Life: Letty's mother disappears, but the family finds a note from "Neville" (the name of her ex-boyfriend) agreeing to pick her up. It turns out Neville is the husband of someone she was visiting to discuss a business venture, but she'd accidentally left his note to her on the table, rather than the note she had written for her family.
In The Teenage Worrier's Worry Files, Letty meets a man named David who is a cross-dresser. She discovers letters between David and her father, describing plans to go on a trip together; and believes they are having an affair. David was actually helping him research for a new novel about a cross-dresser.
In another book, Letty visits her friend Hazel's house to send her ex-boyfriend an email (since Letty doesn't have internet access at home.) His new girlfriend intercepts the message and writes a reply warning Letty to stay away from him, but Hazel's mother finds it and assumes her husband is having an affair. Hazel isn't happy with Letty.
In The Teenage Worrier's Christmas Survival Guide, Letty asks to borrow a dress of her mother's to wear to a Christmas party, but the dress turns out not to belong to her mother, leading to an argument between her parents over whose it is (though it is suggested her dad isn't really cheating.)
In Smallville, it once happened with Lana's biological father's wife. She follows Lana and insists that it's not what it looked like: the other man she was talking to was NOT a love interest...however, he was a divorce attorney. Ouch (they do end up salvaging their marriage though).
iCarly: Tasha and Freddie, mistaken by Gibby, in iEnrage Gibby.
In The Twilight Zone revival, "Love is Blind", this is part of the big twist: it turns out the protagonist's wife isn't seeing another man, she's meeting with an old friend to try and decide what to get for the protagonist's birthday. If the stranger of the story hadn't intervened, the protagonist would have shot her.
Louise finds out George has been giving money and presents to the people in a certain apartment every Christmas, and wonders if he's having an affair or secret children she doesn't know about. It turns out he's just been anonymously helping the family who lives in his old apartment, since he remembers how hard his family had it when he was a kid.
Louise has told George she doesn't want him to work late anymore. George sneaks in from working late and is fact building another store. When caught by Louise, he lets her think that he's having an affair rather than working so much.
Louise catches George in a hotel room with another woman. It's a long story, but it's completely innocent. However, when he tells her that there's nothing going on between him and the woman, she believes him instantly because she notices that he doesn't display the physical tic that usually tips her off to when he's lying.
On Parks and Recreation, Ann thinks Chris is cheating because he's so distant, so Leslie investigates. It turns out he's not cheating. He's distant because he dumped her a week earlier.
Happens in a Tales from the Crypt episode. A jealous and insecure husband overhears his wife planning something with his best friend. As he overhears more of the planning, he becomes more and more suspicious. He ends up killing both and, dragging her body behind him, stumbles into a surprise party planned, where she was going to tell him she was pregnant with their long-awaited child.
Played with in Spaced: Marsha, who believes Tim and Daisy to be dating (when they aren't, but merely pretend to be to rent her flat), believes Tim to be 'cheating' after seeing Tim kissing his girlfriend Sophie. This leads to a confrontation where Marsha threatens to reveal the 'truth' to Daisy; however, Tim - unaware that she has seen him, and believing that Marsha is actually talking about a birthday cake Tim has arranged as a surprise for Daisy, completely misinterprets Marsha ("But... you'll spoil the surprise."). Chaos, not unnaturally, ensues.
Done on Psych, but not as a major plot point. Shawn's date confesses that she has a boyfriend, but plans to dump him because he's cheating on her. When she lists the evidence, Shawn realizes that her boyfriend isn't cheating; he's actually planning to pop the question.
James Wilson on House gets accused of this by House every time he so much looks at a woman (to be fair, he did cheat on two of his wives and told them about it later). This trope is actually the entire B-plot of "Fools for Love", in which it's subverted - it's not Wilson that Nurse Wendy is dating, it's Foreman.
In another episode, House and Stacy, his ex, are washing dishes at her place when her husband comes in. House (acting surprised): "It looks like we're washing dishes, but we're totally having sex."
There's an episode where a married couple is found to have genital herpes. The wife, naturally, is furious, assuming her husband to be unfaithful. He denies it. House then proposes that one of them could've picked it up in a public toilet. The wife claims she knows about it and always covers the toilet seat. The husband claims he never knew. All seems resolved... then House points out that the husband agreed way too quickly. Turns out the guy really is a cheater who was just trying to avoid the reveal by accusing his wife of being a cheater (not sure what his "best case" scenario was there, that she would reveal it was true?)
Rachel and Phoebe see Chandler meet with an attractive blonde lady and follow them out to a house in Westchester where the two of them spend almost an hour together. They, Joey and Ross leap to the conclusion that Chandler is cheating on Monica when in reality the lady was a realtor who was showing the house. Kind of an Idiot Ball moment, since they immediately assume the worst about their friend. Inverts the conventional trope as Monica doesn't believe for a minute he's cheating and figures out who the woman actually is.
Joey also mistakes Monica for cheating when he see's her hiding a guy in her apartment. ("I told you, you shouldn't have married someone so much hotter than you!") Again inverted as Chandler doesn't think Monica is cheating...because he's the one she's hiding.
Used in an entirely non-comedic way in Battlestar Galactica. Cally spots her husband Galen having a long and intimate talk with Tory at a bar and accuses him of having an affair with her. He can't exactly tell her that he was really discussing that both he and Tory happen to be Cylons. And when Cally spies on them and discovers they are Cylons, Tory kills her. Galen kills Tory when he finds out. Also, Cally wasn't just imagining things because Tory and Galen were engaged in a past life and a bit of attraction was coming out in the bar.
Early in the first season of Alias, Francie suspects that her boyfriend, Charlie, is cheating. Sydney and Francie follow Charlie and find him meeting with an attractive blonde woman, into whose car he loads a bag, before driving off with her, while he has told Francie he has a law review. It turns out that she is his partner for a MUSICAL act, and he wants to be a singer.
In Jeeves and Wooster, this happens to Gussie Fink-Nottle all the time, primarily in the first episode of season 2. This is especially bad, because if Madeline broke off her engagement to him, then Bertie would have to marry her. Either that or explain that he would rather die, which would be bad manners.
In season 2, when Dexter and Rita were about to, er, do the deed, Dexters' NA sponser calls... and mentions their road trip, where he confronted his mother's killer, and then they slept together (but did not have sex.) Unfortunately, he let the machine get it, meaning Rita heard everything... and thus, suspected he was cheating. Later on, Lila (the sponsor) and Dexter do have sex, but only after Rita says that their relationship is over. Which apparently she didn't mean, because you know how women can be... so irrational when they think their perfect man is sleeping with another woman. Dexter then confesses to having sex with Lila. Sort of. When Rita asks him to tell her if Dexter had sex with Lila that night, Dexter replies "No... Not that night."
Played with in season 3. At first it's played straight when Miguel's wife suspects him of cheating... when, in fact, Miguel can't say where he was because he was committing a murder. Then later his wife catches when he did actually decide to rekindle an old romance, albeit for ulterior purposes. Dexter knew about it and was the one responsible for getting Miguel's wife to LaGuerta's.
On My Own Worst Enemy, Tom's wife begins to suspect that he's cheating on her when she finds out he was lying about being on a business trip. In fact, Tom's other personality, Raymond, was off on a spy mission, but since Tom doesn't know this, Raymond's bosses have to alter his memories to make him believe he really was cheating. They were actually sort of rekindling their relationship... then she witnesses Raymond (she thinks it's Tom) shooting someone at night.
On Monk, Stottlemeyer asks Monk and Natalie to follow his wife, whom he suspects is cheating. They take pictures of her out with another man, and Stottlemeyer confronts her. The good news: she's not cheating. The bad news: the man is a divorce lawyer...
Another episode, "Mr. Monk and the Bully," has Monk and Natalie follow another client's suspected cheating spouse - he suspects it based on her clothing. They follow the spouse, and catch her in a bar with someone else. The next day, he turns up dead. In the end, it turns out that the spouse was not cheating, and Monk and Natalie were following her identical twin sister.
In the TV show The Invisible Man, there's one episode where the main characters find a man who faked his death by showing his girlfriend a picture of one of the Agency's scientists, an attractive blonde, and claiming the man was having an affair with her. They also plant other signs—lipstick and perfume on one of his shirts, etc. The girlfriend storms off to confront the supposed cheater, leading the main characters right to his hiding place.
Just about every second episode of Bewitched revolved around Samantha mistaking Darrin for doing this, with Endora egging her on.
Robin on How I Met Your Mother has mistaken two of her boyfriends for cheating: Ted in season two (she had heard second-hand stories of Barney claiming to be Ted) and Barney in season five (who was actually sneaking around to learn more about Robin so he could be a better boyfriend to her).
Also with Ted's parents with a slight twist. Barney takes a picture of Ted's father making out with a girl, leading Ted to furiously confronting his father. However, it turned out that his parents had been divorced for almost ten months and separated for two years, intentionally not telling their son because they feared the news would crush him.
It's also revealed that Lily has invoked this a few times when she has disapproved of woman that Ted has been in a relationship with by leaving around an earring for the women to find.
In The West Wing, season 7: Bruno and Vinick find out that Santos discreetly gives a certain amount of money every month to a woman whom he had hired while he was the mayor of Houston, and that the woman has a seven-year old daughter. Bruno and Vinick debate letting the public know about this. Vinick eventually confronts Santos about this; it turns out that Santos's presumed daughter is actually his niece and that he didn't cheat on his wife.
On Chuck the titular character begins dating his college sweetheart, Jill, who walks in on him and Sarah showering (they were trying to get a "poisonous" powder off of their bodies. It turned out to be Hi-C mix). He explains the situation and she is all-too-willing to believe him. This might have had some thing to do with the fact that Jill was evil and using Chuck.
Additionally, after Bryce got Chuck kicked out of Stanford (to prevent him from being recruited for a dangerous mission), Chuck finds out that Bryce is also sleeping with Chuck's girlfriend Jill. Then it turns out that Jill made it up (on orders from her Fulcrum handler), and Bryce never bothered to set the story straight.
In Castle the Victim of the Week was killed over a girl in Cuba, leaving behind a very pissed off wife. Until she learns the girl in question was the victim's daughter. A double whammy followed, as the wife confronted the husband's best friend about the girl, who had no idea what was going on and was trying to calm her down and comfort her... when the husband walked in on them. Guess what conclusion the husband leapt to.
Harper: Oh, everything's fine. Turns out my dad was just sleeping in the car.
Subverted in Cheers when Kelly finds Woody holding another woman (a pre-Friends Lisa Kudrow!) and immediately jumps to the right conclusion:
Kelly: Don't make excuses, Woody. Now I see what's going on. You're busy every night and you won't tell me why. I walk in here and find you two kissing in the back room of a bar. It all adds up. You're in a play and you didn't even tell me.
Woody: Wait, maybe I was just cheating on you!
In the Tales from the Darkside episode Florence Bravo, a woman eavesdrops on her husband and spots him talking to an attractive woman. As soon as she leaves in anger, it is revealed that the woman is simply a lawyer. Unfortunately, a few minutes after the lawyer leaves, the wife returns and shoots her husband, oblivious to the truth.
On Bones, a man with a history of infidelity is killed by his own daughter. His killer was furious that he'd apparently resumed fooling around, unaware that the young woman he'd been meeting on the sly was his illegitimate child.
During another case Booth and Bones see Sweets' girlfriend trying on a wedding dress and hugging a man. This leads them to assume that Sweets is the other man in this scenario. It turns out she was just the bridesmaid filling in for the bride since they had the same build.
In Eureka, Henry's cosmically retconned wife thinks his lack of intimacy is because he is seeing another woman. When she asks Jack about it, he says, "Oh, no, no, that's the least of your problems!" The real reason is because he's from a timeline where they were never married.
In the Warehouse13 episode "Where and When", it turns out the guy they think has this week's Artifact is meeting his secretary to plan a takeover of the magazine they work for. Too bad he neverlet his wife in on the plan. Part of it has to do with how the guy treats women. Everybody assumes him to be a mysogynistic Jerk Ass. It turns out that's just a cover he's putting on for the magazine big shots. He actually thinks very highly of capable women and has no problem with them holding his positions in the company. His attitude is actually pretty ahead of its time (the episode's set in the 1960s.)
In UFO, a flashback to the founding of SHADO has Straker in secret meetings with his new staff. His wife grows suspicious about his late night meetings and discovers him meeting with a female crewmember and makes makes the wrong assumption. Because of the secret nature of SHADO, he can't tell his wife what's going on so she leaves him.
In the Hong Kong drama The Drive Of Life, one character saw her friend's husband having lunch with another woman and both were quite close to each other (such as the woman helping the husband try on a jacket). After the character leaves to tell her friend, we find out that the woman was an old friend of his and the jacket she helped him try on was for his son.
The central focus of the first Kamen Rider Double Returns mini-film is that Ryu Terui/Kamen Rider Accel is framed for murder...and supposedly has run off with a mysterious woman, which infuriates his new wife Akiko and has her running to file divorce papers. (In reality, she's a pickpocket he was in the process of arresting when everything went to hell; they're handcuffed together and he lost the keys.) Not at all helping is the fact that another character is playing with Akiko's worries for a laugh and because she herself has a crush on Ryu; she's the one who gives Akiko the divorce papers, earning a What the Hell, Hero? from her grandfather.
On Glee, the club's best dancers Brittany and Mike are tasked with coming up with a dance number for sectionals. All's fine and good until Mike's girlfriend Tina confides in Brittany's boyfriend Artie note who happens to be her ex, go figure that she suspects Mike and Brittany might be cheating on them with each other. When Artie asks Brittany if she wants to spend more time with him, she avoids the question, saying that she has to rehearse with Mike. Tensions run high all the way to right before the actual competition, and it turns out Brittany didn't want to talk to him because she lost the "magic comb" he'd given her as an encouragement token and wasn't really cheating on him.
Later in the season, a blind item in the school newspaper implies that Quinn is cheating on her current boyfriend Finn with her ex-boyfriend Sam note whom she only left in the first place because Finn pushed her into cheating on him. Yes, Finn is a hypocrite., sending Rachel and Finn on a stakeout of the motel where they are supposedly hooking up. While spying on the motel they spot both Kurt and Quinn leaving Sam's motel room, and come to the conclusion that both of them are cheating on their boyfriends with Sam. Turns out that Sam's family lost their home after his father lost his job, and all of them - mother, father, three children - are living in that one hotel room; Quinn was helping Sam babysit his younger siblings and Kurt was helping out by giving Sam some second-hand clothes.
Happens on The Good Life when Tom and Barbara discover that Margo has not been taking riding lessons like she claimed. They try desperately to keep the secret from Jerry. It turns out she has actually secretly been going to weight loss classes.
In Noah's Arc, Alex suspects Trey may be cheating with his friend Guy. Early on he investigates it and finds he was completely off base. A subversion comes up though in that later we learn Guy was secretly pursuing Trey all along, without Trey's knowledge.
Boy Meets World had the "kids think that one of their parents is cheating" version. They find out that their mom is sneaking off to have a romantic dinner with a mysterious man... who turns out to be their dad.
Another episode had the parents checking into a hotel to celebrate the wife's pregnancy. At the check-in desk, the husband impulsively shares his news with a random woman, who gives him a congratulatory hug. Sure enough, someone sees this and leaps to the wrong conclusion.
On Rumpole of the Bailey, Mrs. Rumpole (Hilda, a/k/a "She Who Must Be Obeyed") thinks her husband's cheating on her. Actually, He's taking dance lessons because she's mentioned being annoyed that he can't dance with her in the past. At the end, it's a CMOH when they dance together.
CSI: Crime Scene Investigation did this in "Genetic Disorder" with Doc Robbins' wife. Although Brass and Hodges refused to believe the couples' assertions they were faithful, it was proven to be a frame up made to look like she'd been cheating and a random one at that.
CSI NY Danny was Mistaken for Cheating by Internal Affairs. One of his rookie cops shot an unarmed man while they were out for a drink-two guys attacked, and one did have a gun, but she shot the other one. Scared for her career, she told IA Danny told her to lie because they were having an affair, and IA was highly suspicious of a surveillance tape that showed her cozying up to Danny at the bar. Danny vehemently denied it, naturally, but Lindsay had to pressure her to tell the truth and admit what really happened.
Brothers and Sisters had the complication of Robert's presidential campaign. An investigator took suspicious photos of him entering a woman's home and showed them to a member of his staff, who happened to be his brother-in-law Kevin. Kevin is torn over his job and loyalty to his sister. In the end it turns out that the woman was actually his therapist.
Done this way and that on all versions of Law & Order. One noteworthy example has the detectives confronting the husband about his clandestine behavior only to have it turn out he was having an affair with his WIFE and that the sneaking around was an attempt to put the spice back into their marriage. Another had the SVU detectives confronting a man they assumed to be a dead woman's lover, only to have it turn out he was her husband's boyfriend (it's a long story, but suffice it to say, everyone was aware of what was going on, and no one was guilty of murder).
Big Love does an interesting version of this as well. Husband Bill and his first wife Barb begin an "affair" (he is not supposed to sleep with her on nights designated for the other wives). Second wife Nikki notices his suspicious behavior, concludes that he is courting a fourth wife and is actually pleased, as she thinks it means she will gain a friend and an ally in her clashes with Barb and third wife Margene. When she discovers he's fooling around with Barb, she's hurt and disappointed.
On The Dick Van Dyke Show Rob thinks that his coworkers Buddy and Sally are having an affair, but it turns out they're just moonlighting as a comedy song-and-dance act.
In another episode Jerry the best friend and next door neighbor is suspected of cheating on his wife when he's seen lunching in a restaurant with another woman. She turns out to be a fellow dentist.
Soap: Burt walks in on Mary and her English professor having a snog on the couch and assumes they're having an affair (which is what he was afraid might happen if she went back to school). But it's really her professor throwing himself at her. Right after Burt leaves she slugs the prof and throws him out of the house.
A drunk Burt then goes to his secretary Sally's apartment. Sally had been throwing herself at Burt. He sleeps off his drunkenness but Sally makes him think that he had sex with her, so now Burt mistakes his own behavior for cheating.
In an episode of Drake & Josh, the eponymous characters think Walter is cheating on the mom. It turns out he was meeting with someone about a new job and wasn't sure about so he waited to tell his family. Drake and Josh screwed up his chances of getting the job, though.
In an episode of Cold Case, the wife of the victim had suspected her husband was having an affair with another woman who was also involved in a communist movement like him. She felt that she as a simple housewife couldn't compare to this exotic foreign woman. The woman in question would later reveal to Detective Rush that while she confessed her love for the husband, he remained faithful to his wife and family.
In another episode, the detectives learn that a beautiful young black woman had visited the white murder victim at his office shortly before he was killed. His reaction toward her—looking very upset that she showed up, rushing her out of there—plus the revelation that he was sending money to her makes the detectives jump to the conclusion that she was his mistress. Only to have it turn out that she was his SISTER. He was passing for white and her appearance could have blown his cover.
On Nashville Teddy Conrad is running for mayor of Nashville and is photographed comforting a woman who is not his wife. He is not cheating but he can hardly explain to the voters that he was comforting the woman because she was his accomplice in a fraud he once committed and she was scared that they would be exposed. In turn Teddy's wife is often suspected of cheating on him because she is still very close to Deacon Claybourne who was her boyfriend before she met and married Teddy.
In the Series 3 episode "One Night Stand", a practical joke-loving co-worker of Martin's arranges for him to find a strange woman in his bedroom one morning on a business trip. Martin is horrified to discover his "indiscretion" and immediately confesses to Ann, who becomes cold toward him until Paul tricks the joker into admitting the truth in front of her.
In the Series 4 episode "Jumping to Conclusions", Hilda sees Ann climbing over the fence into Paul's garden for a study session for her Open University course. She concludes that Paul and Ann are having an affair, and word works its way back to Martin. Martin decides to leave Ann to what he sees as a happier life with Paul, but is set straight before he can make the separation permanent.
Invoked and played with a lot in 'Allo 'Allo!. Edith often catches René making out with one of his waitresses or some other woman, and each and every time he just comes up with a lame excuse to make her believe it's this trope. The kicker of course is that he is very much cheating on her. And she buys it every single time. He only drops the act in the final episode.
In one episode of Goodnight Sweetheart, Gary becomes convinced his 1940s wife is having an affair with his next door neighbour ... who happens to be Noël Coward.
In That Seventies Show, Donna finds a pair of panties in Eric's car. She logically gets angry and believes Eric is cheating on her with a girl named Shelley. The panties belong to Donna's mother Midge... since she and Bob had some "fun" in there.
On ER, Elizabeth discovers that husband Mark went to New York when she sees the charge for a plane ticket and hotel room on his credit card statement. Given their current estrangement and his Unresolved Sexual Tension with friend Susan Lewis, she naturally comes to the conclusion that they're having an affair. Only for the truth to be even worse—Mark went to New York to visit the neurosurgeon who had previously operated on him, because his lethal brain tumor has returned.
In the Brazilian telenovelaAnjo Mau ("Evil Angel") and its remakes, the father of the baby boy that the Gold DiggerVillain Protagonist babysits is accused of cheating on his Clingy Jealous Girl wife since he periodically visits someone else's house and says nothing about his reasons to do so. The women he visits are actually his mother and sister.
A strange variant happens on The King of Queens. Carrie is upset that Doug loves Spence's girlfriend Becky's cooking, since Carrie herself is a mediocre cook. She gets so upset that she bans Doug from eating Becky's cooking or even going to her house. When Carrie confides in her father Arthur about it, he assumes that "eating Becky's cooking" is a euphemism for "cheating". Later on, when Doug gets angry at Deacon for eating Becky's food, Deacon asks if they were sleeping together.
The Without a Trace episode "Midnight Sun" had a supposedly loving family man disappear, only for witnesses to report seeing him on several occasions at a diner in the company of an attractive young woman. A trace on the woman's credit card reveals that she is in fact a deputy U.S. Marshal, and the man's handler in Witness Protection.
In the Get Some In! episode "Kit", Drill Sergeant Nasty Corporal Marsh buys a box of hankies from Teddy boy aircraftman Jakey Smith, who secretly has dozens of boxes he is selling at a huge profit. He sells another box to LAC Hodder, the quartermaster at RAF Skelton, who gives them to his girlfriend. When Marsh shows up with Aircraftman Second Class Richardson, who has lost his kit and needs a replacement, he finds one of the hankies and assumes it must belong to Alice, and he beats Hodder up in retaliation. However, when he discovers that Alice still has all the hankies in her box, he punishes Smith for being a spiv and Richardson for doubting Alice's fidelity.
In House of Anubis, Eddie begins to believe towards the end of season 2 that Patricia is seeing someone else because of how often she disappears without telling him why, and he suspects it might be Alfie. He eventually follows her, Fabian and Alfie when they leave for the barn and finds out the truth.
Patricia later believes Eddie himself was cheating on her season 3. In the beginning, she and Fabian believe that Eddie and Nina had a summer romance and that's why Nina didn't come back, because of some emails they found on Eddie's computer and the fact that he had Nina's locket. It turned out that Nina gave him the locket because he might need it, and the emails were about the whole Chosen One and Osirian thing. Later on, Miss Denby tricks Patricia into believing Eddie is cheating on her by placing fake messages on his laptop and manipulating her into reading them. It doesn't get resolved too easily, as Patricia turns into a sinner for getting upset over this, which was the whole plan. Both times, this was Played for Drama rather than laughs.
The Mecano song "Hijo de la Luna," covered by Sarah Brightman. A gypsy woman makes a deal with the moon to find her a husband, as long as she agrees to give up her first-born child in return. The baby ends up as pale as the moon. Her husband ends up killing her, and the moon gets its child.
Played darkly in Vocaloid's "The Tailor Shop on Ebizaka" where the tailor was upset that her lover hasn't been coming home to her and finds out he was seeing three different girls on different occasions. She kills the three girls and took their clothing/accessories thinking it would make her beautiful to her lover. It turns out that her "lover" doesn't even know who she is and the three girls were his wife and two daughters. She then kills him too, offended by his lack of recognition.
Mythology and Religion
In the Biblical account of the Nativity of Jesus, Joseph initially assumed that Mary's virgin pregnancy was the result of infidelity, until an angel explained the truth to him.
One Jewish legend says that David was conceived due to a Bed Trick after his father, Jesse, stopped sleeping with his wife; as a result, Jesse assumed David was the product of an affair throughout his childhood.
A series of FoxTrot strips has Andy convinced that Roger is cheating on her when he's actually at work. ("Listen — hear that squeaking? You know how my chair squeaks." "Sounds like bedsprings." "Fred, get in here and talk to my wife.") In the last strip, she apologizes for being paranoid and they hug...at which point she plucks a hair from his jacket and asks whose it is. (No, Roger wasn't cheating; he's just balding.)
Fred's response to being called over is "Not again...", implying that Andy does this with some degree of regularity.
Some of the villains of Ravenloft have this trope in their backstory. Anton Misroi caught his wife in the arms of another man, whose shoulder she'd been crying on because Anton was so cruel. Romir Hiregaard leapt to conclusions after seeing his wife and a family friend arm in arm, unwilling to believe that he'd interrupted a waltz lesson. In each case, the wives wound up murdered, and the villains, cursed by the Dark Powers.
It's played extremely seriously in Othello, when the title character believes that he overhears his friend Cassio laughing about sleeping with Othello's wife. Cassio is in fact talking about another woman entirely, and the whole thing had been set up by the villain of the piece in order to mess with Othello's head, but nobody discovers this in time to prevent the usual Shakespeareantragedy ensuing.
This happens to Fiyero and Elphaba in act 2 of Wicked. Somewhat subverted in that Fiyero most definitely did want to be cheating on Glinda at the time.
Lord Windermere in Lady Windermere's Fan. The woman his wife suspects him of is actually blackmailing him with a secret he wishes to protect his wife from. She redeems herself later on, though.
In Galaxy Angel Moonlit Lovers, Ranpha suspects Tact of cheating on her with Chitose because she walked in on their hands touching while picking up papers. He spends much of the rest of the story path trying to win her back, eventually dressing up in a ridiculous pink "I love Ranpha" outfit.
Invoked and played for laughs at the beginning of Duel Savior Destiny. Taiga is currently attempting to get a hundred girlfriends at once and calls one of them by the wrong name, at which point he attempts to play it as a joke with this trope. She doesn't buy it.
In The Elder Scrolls V: Skyrim, Ranmir thinks that his fiance Isabelle "ran off with a thief named Vex", assuming that Vex is male and that Isabelle was cheating on him. Turns out that Vex is actually a woman, Isabelle's old friend, and that she had gone to Vex to ask if she knew of any treasure; Isabelle wanted to fix Ranmir's financial problems and make him happy. She had died in the attempt. The player gets to show Ranmir a note she had written before her death explaining everything.
Sorta happens in Fire Emblem Awakening. When Chrom shares a hug with his Kid from the Future Lucina before they explain their relationship, they get caught... by Lucina's mother, who has no idea of what's going on, and ends up wondering if they're having an affair (The exact reactions are different per girl). They explain the situation, Lucina hugs her still shell-shocked mother, and everything will be solved.
Also, if Chrom and a female Avatar DON'T get married, the Avatar's supports with Lucina will involve the latter accusing the first of either being Chrom's lover or intending to seduce him.
The Wolf Among Us: Beast is concerned by Beauty's disappearances late at night and is worried that she may be trying to hide a dark secret. When he finds her with Bigby at a seedy motel, he assumes the worst and a scrap breaks out. The truth is that Beauty has been secretly working at the front desk to help pay the rent, and Bigby is there to investigate a murder.
In Peter Is the Wolf, Peter gets knocked out and kidnapped by a long time stalker of his. As she is about to rape him, his girl friend bursts in, mistakes the rape attempt for him cheating on her, dumps him on the spot and runs off.
In Skins, Jordan walks in on what seems to be Vinnie making out with another woman.It later turned out the other woman spontaneously kissed him and only because she thought he was dead and was delighted to see him alive but Jordan was so mad she bashed Vinnie over the head with a whiskey bottle.
Subverted in College Roomies from Hell!!!, where Marsha walks in on her boyfriend Mike spanking the monkey - which is to say that literally, he has a pet monkey which he is punishing for having messed up the apartment.
The webcomic Punch an' Pie has this happen constantly to Heather at the beginning of the strip. Her girlfriend Angela seems to think that literally every one of Heather's friends is a possible affair. Angela believing that Polyamory is natural and both being bisexual apparently makes Angela very jumpy about any contact with someone outside the relationship.
Almost happened in the Walkyverse, but was averted by one party being Genre Savvy. Danny is talking with ex-girlfriend Sal and mentions that he is planning to ask current girlfriend Billie to marry him.
Sal: Can I see the ring?
Danny: Hell no. I know how that works. I show you the ring, and Billie walks around the corner.
In Kevin & Kell, when Bruno starts taking classes with fellow herbivores, he meets a rhinoceros named Rachel Einhorn. She is attracted to him at first sight, angering Corrie when she gives him a kiss after saving him from some coyotes and thinking he is cheating on her. It turns out that she, a lesbian, thought he (a "trans-diet" wolf wearing a sheepskin) was a sheep, resulting in him wearing ram horns with his sheepskin from that point on.
In General Protection Fault, Craig is thought to be cheating on his wife with Sharon, as Nick and Dexter spy on the two. Similarly, the German, while taking on Craig's identiy during his business trip to Paris with Sharon, makes advaces on Sharon, but when she rebuffs him, realizes that he has misunderstood their relationship.
The aftermath of such occurs in this short comic, where the boyfriend explains that the other girl he gave a kiss to was just his sister. Then it turns out to be incest. And he had sex with her just after the kiss. And offers to let his girlfriend join and make it a threesome.
In Questionable Content, Dora finds Marten and Faye on the couch in their bedclothes hugging and jumps to the entirely wrong conclusion. The setup starts here.
The Flintstones had an episode where Fred's accidental discovery of a poem dedicated to Wilma, as well as Wilma's strangely secretive behavior, make Fred suspect that Wilma cheats on him. The catch is in the fact that the poem was actually written by Fred himself when he was still a high school student, while Wilma was secretive because she wanted to surprise Fred for his birthday. It doesn't help that a man calls wanting to talk to Wilma, and she acts strangely, like she doesn't want Fred to hear. He becomes suspicious and goes to another phone, to hear her say, "...that's right, Darling, not the only man I ever loved, the only man I ever will love." Fred thinks he's about to lose Wilma to another man. It turns out he didn't get to hear the beginning of the conversation, where the man said to her before she responded, "This is the jeweler calling, on your husband's watch for his birthday, you want the engraving to read 'Darling...'" It was at that point Fred picked up the phone.
In another episode, Wilma suspects Fred of cheating, though with better cause: He's coming home late from work, unusually tired, and when she follows him, she finds him in a romantic nightclub him with a very sexy blonde. The blonde is a charm school instructor, and Fred has been taking courses in dancing, small talk, and related matters so as to make their upcoming anniversary more special and memorable. That night was the final exam/dress rehearsal.
In a third example, both Fred and Barney fall victim to this trope when they decide to take dance classes so they can take their wives dancing. In order to cover for their classes, they join the "Bedrock Volunteer Fire Brigade", which is a front for the men of Bedrock to get away from their wives; since all buildings in Bedrock are made of stone, all fire alarms are false alarms. Once again, once the truth comes out, Fred and Barney are Easily Forgiven by their wives.
Homer Simpson: "I must be the only gullible husband who ever overheard snippets of surprise-party planning, and believed his wife was having an affair!"
Another instant was when Homer was returning from France and sees Marge hugging Flanders (she had invited him over for a family dinner). Subverted in that Homer was Genre Savvy enough to know that it's probably not what it is. Semi-Double Subverted because they had both been thinking about having an affair but resisted it.
And there was that episode where Homer and Marge was giving marriage counselling to an idol and her baseball player husband. When Homer gave the idol a backrub while eating chicken, her husband on the other side of the door naturally assumed with the moans they were making that his wife was cheating on him.
They love subverting this. In another episode Marge listens in on Homer in the next room with his Vegas wife (long story). She hears him says things like, "Oh that's good. Don't stop! Faster, faster! You do this like a pro!" Her response, "Oh no! She's making him a sandwich!" She was.
In yet another episode, Marge writes a novel that has echos of real life. Novel-Marge has an affair with Novel-Flanders, which everyone in the town assumes means Marge has the hots for Ned. Lisa is worried that life will imitate art in another way, since in the climax, Novel-Homer character kills Novel-Flanders and himself suffers a Karmic Death. While Homer does chase Flanders to a cliff, it turns out that he just wanted to ask Ned for advice on how to be a better husband, having realized that he's been a real jerk lately.
Happens to Reef in the Stoked! episode "Sweet, Sweet Meat Cheat". Fin overhears Reef make a comment about cheating on Lo and assumes the worse. He is actually 'cheating' on the vegetarian diet she put him on by eating franks.
The Looney Tunes Show: In "Beauty School", Lola sees a woman (actually Bugs in drag) leaving Bugs' house and getting into Bugs' car and immediately assumes that Bugs is cheating on her.
In one episode of the Justice League, to prevent ex-employee Rex Mason from exacting revenge for turning him into a shape-shifting mutant, corrupt businessman Simon Stagg shows him a picture of his fiancee being tenderly embraced by his old friend Green Lantern (who was actually comforting her after Rex's mysterious disappearance). Overcome with jealousy, Rex promptly goes to attack Green Lantern.
The Amazing World of Gumball: In "The Plan," Gumball, Anais, and Darwin discover an apparent love letter from "Daniel Lennard" to Nicole reading "Your beauty and happiness is important to me," spend most of the rest of the episode drawing up an elaborate plan to stop "him," and find out only in the end that "he"'s just a cosmetic brand.
Averted in Kim Possible. Bonnie, upset at the loss of her boyfriend, forces a kiss on an unwilling Ron. Kim catches them, and, though upset at Bonnie, she completely believes Ron's claim that Bonnie forced the kiss (because, after all, it took years for Ron to kiss her.)
A tragic real life example: Rush Dozier in his book Why We Hate, describes an event when a friend of his separated from his wife after several attempts at reconciliation. The man decided to call his wife before moving to another city and an unfamiliar male voice answered the phone. The man rushed to his wife's apartment (it turned out she was alone), broke down the door and fatally shot her, then himself.
Things can get odd for adult brother/sister siblings who travel or even just go out to dinner together when people who are unaware that the person has a brother/sister sees them out having dinner together or sharing the same hotel room.
Or relatives of any kind. One family found themselves both amused and dismayed by how many of their friends reported to the wife that they had seen the husband out to dinner with an attractive young woman—his daughter from a previous marriage.
Or. . .a woman wrote to Dear Abby to complain about a mysterious young woman who was visiting her married neighbor late at night, being admitted by the husband and later let out. The woman was sure that the man was "sneaking this tart in for sex for his poor wife sleeps upstairs". Reader response suggested that the wife might be fully aware that this young woman was coming over—that she might even be there to see the wife herself and advised the nosy neighbor to butt out.