I thought she was my friend,
but now it's time to diss her!"
Friendships and relationships are similar as they both need trust and honesty to work. Both members of the party need to display these qualities to keep the relationship healthy.
Unfortunately, not all relationships follow through with this. Sometimes, the person you're supposed to be with goes behind your back and hooks up with someone else. While this is already bad enough, it only gets worse when they are going out with one of your close friends.
Situations like this can really break the trust and bond between not only the romantic partners, but the two friends as well. While the victim of this will reasonably be angry at both of them, more of their anger goes towards the friend instead of the romantic partner. This is usually due to how friends are supposed to be the ones that they can lean on for support, which makes the betrayal hurt more.
In some stories of fiction, this would allow for a Love Triangle to occur in the fandom. There would be fans that would support the former love interest, due to how they were hurt by this betrayal. Other fans would root for the new couple as they felt the former relationship was unhealthy and abusive. Then, there would be the fans that would Take a Third Option, and say that all of them were at fault for what has happened and wouldn't take any sides at all.
A situation like this can lead to tropes such as Et Tu, Brute? or We Used to Be Friends, as they both involve a friendship being broken due to betrayal from their friend hooking up with their partner behind their back. Some cases could also apply for Sympathetic Adulterer, depending on how the former relationship was and if the cheated victim was a good person to their ex when they were dating.
This is one possible reason someone would Blame the Paramour; friendship betrayals hurt just as much as romantic ones.
Compare A Family Affair, which involves a similarly personal betrayal, as well as the cheater "hunting close to home".
- Shortly before the start of Days of Love at Seagull Villa, Mayumi learns that her boyfriend is leaving her for her best friend Touko, whom he supposedly got pregnant.
- Kaguya-sama: Love Is War
- Subverted with Kashiwagi. She thought that her boyfriend was messing around with her best friend Maki behind her back, but he was actually just asking her for help in picking out an anniversary present. Double Subverted when it turns out that Maki was actually trying to break them up so she could have him for himself (though she admits to Shirogane and Ishigami that she'd never been able to fully go through with it because she cares too much for Kashiwagi as a friend to break her heart like that).
- Tsubame developed Commitment Issues after her boyfriend cheated on her. It's implied that the other girl in the equation was her best friend, but Onodera doesn't give enough detail when explaining the situation to Miko to make it clear one way or another.
- Fantastic Four: Alicia Masters, a blind sculptress, had been in love with Ben Grimm aka The Thing for years, with her love often a source of strength for Ben in reminding him that there was at least one person outside of the FF who saw the human inside the monstrous exterior. However, the '80s had Johnny Storm aka Human Torch and Alicia grow closer, with them marrying in the FF's 300th issue. Johnny and Alicia getting together was naturally the source of some tension between Ben and Johnny, and proved so unpopular with the fanbase that roughly 50 issues later, it was revealed the "Alicia" Johnny had married was the Skrull spy Lyja while the real Alicia was rescued and picked up her romance with Ben.
- Runaways (Rainbow Rowell): Victor Mancha falls in love with Gert Yorkes, the ex-girlfriend of his best friend Chase Stein. Chase is not okay with it at first but concedes that it's been two years since he and Gert were together, and thanks to time-travel shenanigans, there is now a considerable age difference between them.
- Trouble (Marvel Comics): A teenage May and Mary worked together at a resort for a summer job. At the resort, they meet Ben and Richard, after which May starts dating Ben and Mary starts dating Richard. Since Mary doesn't want to have sex with Richard, he has sex with May instead, while she is already having sex with Ben. She ends up pregnant with Richard's baby, since Ben was revealed to be sterile.
- A.M.I.: Artificial Machine Intelligence: Cassie's boyfriend, Liam, is cheating on her with her best friend, Sarah. In fact, Sarah has actually slept with every boyfriend Cassie ever had.
- Bridget Jones's Diary: Daniel Cleaver tells Bridget that his friend Mark Darcy did this with Daniel's fiancée, causing the present distance between them. It turns out to be the opposite: Daniel was the one who cheated, with Darcy's wife. Bridget's perspective on Darcy completely changes once she discovers this.
- Death Becomes Her: The plot is kicked off with Madeline stealing her best friend, Helen's, fiance, Ernest. Years later, Helen returns, somehow rejuvenated, and begins a plot of revenge, seducing Ernest to kill Madeline and have him to herself.
- Don't Hang Up: One of Mr. Lee's attempts to turn Sam and Brady against each other is revealing that Brady had slept with Sam's girlfriend, Peyton.
- The Dry: Luke, Aaron, Ellie, and Gretchen are all close friends. Luke is dating Ellie while Aaron is dating Gretchen. But Aaron and Ellie are sneaking around behind Luke's back. Not that Luke would notice since he's doing the same to Aaron with Gretchen. This becomes a problem when Ellie is found dead in the river, with everyone else in the group having a possible motive for having killed her.
- In The Fabelmans, Mitzi is having an emotional affair with her husband's best friend and subordinate Bennie.
- At the beginning of Hope Floats, Birdie is given the news that her husband Bill is having an affair with her best friend Connie and is planning to leave Birdie for her. Not only is the reveal painful, but it's also humiliating — the news is broken to her on an episode of The Ricki Lake Show, to which she was brought under false pretenses.
- Lake Placid: In the first film, Kevin breaks up with his employee Kelly. Heartbroken by this, she goes to friend and co-worker Myra for support, who was actually with Kevin behind her back. This starts off the plot of the film as Myra suggests that Kelly goes to investigate a monster attack in Maine, where Kelly finds herself happier without their presence.
- The Last Boy Scout: Joe's Establishing Character Moment comes when he discovers his wife Sarah is cheating on him with his best friend Mike, and promptly threatens them at gunpoint. Mike is killed shortly after when he's blown up by a car bomb, but the affair comes back to bite Joe when the police figure it out and suspect Joe killed him.
- Naked Lunch: William walks into his apartment to find his wife Joan having sex with his friend Hank. William is seemingly as non-plussed about this as he is about everything; however, he shoots Joan shortly afterwards in what appears to be an accident. It's left ambiguous if he killed her as revenge or if it truly was an accident, though William is convinced it's the latter.
- Happens twice to the obstetrician in The President And His Granddaughter. He considers New Year's Eve to be his unlucky day, and his diary entries for that day from two different years read "Discovered my wife in the arms of my friend". He clarifies these were two different wives, but the friend was the same.
- Revenge of the Sith: After the Jedi are betrayed and slaughtered en masse throughout the galaxy, Jedi master Obi-Wan Kenobi confronts Padme Amidala to find out if she knows where Anakin Skywalker, who had led the assault on the Jedi Temple and personally slaughtered several younglings, could be. She denies knowing anything, but Obi-Wan can sense she's lying and stows away on her personal starship as she goes to Mustafar, where Anakin is waiting. When she confronts Anakin about the attack and the murders, Obi-Wan reveals himself. An enraged and unstable Anakin accuses Padme (his secret wife) and Obi-Wan (his best friend and mentor) of betraying him; the subtext implies he's accusing of them of infidelity too. Anakin is thoroughly mistaken, but he Force Chokes Padme and engages Obi-Wan in a duel. By the end, Obi-Wan has fled into hiding, Padme dies after giving birth, and Anakin, grievously burned, is sealed into an armoured suit as Darth Vader.
- The Room (2003): A central plot point of the film is when Lisa, Johnny's fiancée, sleeps with Mark, Johnny's best friend. When Johnny finds out about it, he becomes so heartbroken that, by the end of the film, he kills himself.
- Something Borrowed: Rachel sleeps with Dex, who is engaged to her best friend, Darcy. The two spend the film dancing around each other awkwardly after.
- Spider-Man: Mary Jane would have the famous upside-down kiss with Spider-Man. The problem is that she's dating Harry at the time and (unbeknownst to her) Spider-Man is Peter Parker, his best friend. On the other hand, Harry is shown to be increasingly insensitive and controlling, so it's easy to sympathize with her.
- Harry later attempts to convince Peter that it's happened in reverse in Spider-Man 3, by coercing Mary Jane into telling him Peter that she's found someone else, and then smugly informing Peter that he's the 'someone else'. By this point, of course, Harry doesn't exactly see Peter as a best friend- but Peter doesn't know that yet.
- Tangerine: Sin-Dee spends most of the movie hunting down her boyfriend Chester after he cheated on her with a cis girl, but as soon as she reunites with him he is Easily Forgiven and everything's back to normal. However, it's revealed that Sin-Dee's best friend Alexandra also slept with Chester, and both relationships seem over, though the film ends with a bit of a (gender-flipped) Bros Before Hoes scenario as Alexandra gingerly starts repairing her relationship with Sin-Dee.
- Talladega Nights The Ballad Of Ricky Bobby: After Ricky is fired from being the top car racer for Dennitt Racer, Carly leaves him for Cal, since Cal will now be the top racer. Cal is also upset at Ricky for never letting him win a race when Cal always helped him win.
- Trading Places: At the start, Louis Winthorpe has been engaged to Penelope Witherspoon and was planning to marry shortly until he's wrongly arrested for drug dealing and embezzlement as part of a bet by his employers, the Duke Brothers. Later, while trying to explain everything, she seems to trust him. That is, until they're interrupted by Ophelia, a hooker who was hired to make it seem like Louis was cheating on Penelope with her, after which she doesn't want anything to do with him. Later, when Louis finds her again, he sees that she's since started dating Todd, his friend from the gentlemen's club, who also doesn't want anything to do with him.
- Unfriended: Blaire cheated on Mitch twice with his best friend Adam. Mitch takes it especially hard because Blaire had refused to have sex with him.
- American Gods: Shadow is released from prison early, due to his wife Laura's sudden death. At her funeral, he learns that she had been having an affair with his best friend during his incarceration. The car crash was from her pleasuring him in the passenger seat.
- American Psycho: Patrick is "almost-certain" his girlfriend, Evelyn, is having an affair with his best friend, Timothy Price, but Patrick isn't all that concerned because he's having his own affair with Evelyn's best friend, Courtney.
- Avalon High: Based on The Once and Future King and other Arthurian Legends, Will's girlfriend Jen cheats on him with his best friend Lance. New girl Ellie discovers this by accident and is asked to keep this a secret in order to keep Will focused on an upcoming football championship that could determine his future. Ellie and Mr. Morton draw parallels to the Arthur-Guinevere-Lancelot love triangle of legend, believing this (among other things) to be a sign that Will is King Arthur reborn. Unlike Arthur, Will ends up taking the betrayal better than expected because he's already transferred his affections to Ellie. A different betrayal is what almost breaks him.
- The Bone Clocks: The main character, Holly found her best friend Sheila in bed with her boyfriend. This quickly ends both relationships, and Holly avoids both the boyfriend and Sheila for the rest of her life.
- The Girls Series: In Girls In Love, Magda kissed Ellie's boyfriend Russell behind her back. But they do eventually make up.
- InCryptid: Subverted in the Alice and Thomas prequel shorts. Alice has a crush on Thomas but walks in to find her best friend Mary kissing him in front of another woman. It turns out the other woman is Gwendolyn Brandt, Thomas's betrothed, and Mary was pretending to be his wife so Gwendolyn will leave. Unfortunately, Thomas can't tell Alice it's Not What It Looks Like without revealing the ruse, so she runs away in tears.
- The Kite Runner: From a retelling of events, Amir learns that his Baba's friend, Hassan, and his wife had two failed pregnancies before having Sohrab. Only that Sohrab is actually Baba's child since Hassan is infertile.
- T.H. White's The Once and Future King was a novel in modern English based on the life of King Arthur. White leaves no doubt that Sir Lancelot was a good friend and loyal knight of King Arthur, nor that Lancelot and Queen Guinevere were engaging in adultery. The conflict when Arthur must judge them both after they've been caught in the act is a dramatic high point.
- Steve Higgs' Patricia Fisher series is kicked off when the protagonist catches her husband in bed with her best friend. She immediately sets off on a cruise and gets embroiled in one mystery after another while aboard, everything from jewel theft to murder.
- Pretty Girls: Claire cheated on her husband Paul with his business partner and close friend Adam. Although it later turned out that Paul never really had friends, being a serial rapist and murderer, and he was never bothered by Claire's infidelity except by how it embarrassed him.
- By the third act of Skippy Dies: Lori's best friend Janine manipulates Lori's sort-of boyfriend Carl into sleeping with her instead. This is expected to be a big betrayal, as Janine sets up a dramatic confrontation where she reveals her secret love, but by this point, Lori no longer cares as she has been hospitalized for an eating disorder and has her mind on other things.
- In 10 Things to Do Before I Die, Ted kisses his best friend Mark's girlfriend, Nikki, before Mark and Nikki break up. Somehow, they all remain friends afterward, even when Ted and Nikki start dating properly.
- Beverly Hills, 90210: While Brenda was spending a summer in France, her boyfriend Dylan and her best friend Kelly started falling for each other. Later, when Kelly and Dylan were a couple, Kelly started falling for his best friend, Brandon.
- The Big Bang Theory: Penny once had a best friend who stole her boyfriend, and upon seeing them still together while on a date with Leonard, determines to out-romance them. The ex proceeds to propose to the best friend, which Leonard attempts to match, but given the frequency of his proposals, Penny says no before he even has the ring out of his pocket. And yes, he carries it around just in case.
- Bob Hearts Abishola: Bob's first marriage fell apart when his ex-wife Lorraine had an affair with his best friend Gary. As of the start of the series, he is no longer speaking to either one of them.
- The Brittas Empire: Carole's backstory involves her husband cheating on her with her best friend and running off to Spain together, leaving her financially destitute.
- Castle: In "A Deadly Game", evidence points to the friend and business partner of the victim having motive and opportunity...until he offers as his alibi that he was not murdering his friend...because he was sleeping with the guy's wife at the time. The wife confirms. And the neighbors, as well (implying The Immodest Orgasm by pointing out it was a hot night and they'd left the windows open).
- Creepshow: In the episode The Man in the Suitcase, after finding out about the titular man's ability to uncontrollably produce gold while in pain, main character Justin's ex-girlfriend quickly comes back to him, and just as quickly begins sleeping with his roommate behind his back.
- Criminal Minds: The killer in "The Slave of Duty" snapped when he found out his fiancee was cheating on him with his best man. He then started stalking women who reminded him of her and forcing them to act as his girlfriend, killing them if they went off-script.
- Dark (2017): Upon returning to school after having spent months in a psychiatric ward, Jonas discovers that Bartosz, his best friend, became a couple with his girlfriend, Martha, while he was gone. This ends up stressing Jonas and Bartosz's friendship. Jonas' return results in his relationship with Martha being revived, but he dumps her upon learning that her younger brother Mikkel got trapped in the past and grew up to become his father Michael, making Martha his aunt. Although Martha continues her relationship with Bartosz when Jonas gets trapped in the future, she and Jonas still hold feelings for each other, when he returns 7 months later, even after she learns the truth.
- Dawson's Creek: When Joey and Pacey secretly get together in season 3, Dawson sees it as a betrayal, although he and Joey have been broken up for months after he rejected her.
- In an episode of Dr. Quinn, Medicine Woman, Colleen's best friend Becky is sweet on Richard, but when Becky is sick, Colleen unexpectedly finds herself kissing Richard, which devastates Becky when she learns about it. Meanwhile, in the same episode, some odd circumstantial evidence leads Michaela to think that Sully and Dorothy might be up to something behind her back. In the end, Richard reveals himself to be a budding Lothario and both girls write him off, while it turns out that Dorothy has been secretly teaching Sully to dance so he can partner Michaela at a town social.
- Euphoria: Cassie sleeps with Nate, Maddy's ex, in secret, which is treated as a massive betrayal.
- Friday Night Lights: Jason Street and Lyla Garrity are a long-time couple, planning to marry once Jason finishes college and is recruited to the NFL. When Jason is permanently paralyzed from the neck down by a freak accident during a game, Lyla and Jason's best friend, Tim Riggins, spend a lot of time together trying to figure out how to cope with Street's injuries and help him feel better. This eventually leads to sex. Jason discovers their duplicity when he sees them kissing from the window of his hospital room.
- Friends: Chandler developed feelings for Joey's girlfriend, Kathy. One night when the two are alone they ended up kissing each other. After Chandler confesses to him, Joey gets pissed off at Chandler for doing so. To make it to him, Chandler has to stay in a box to think about what he has done. Subverted when Kathy comes to break up with him and Joey comes to forgive Chandler.
- Ghosts (US): Pete died in The '80s, but hangs around as a ghost into The New '20s, and is delighted when his widow visits the mansion he's haunting. When he finds out she's remarried to his best friend, he's actually genuinely happy for them, reasoning they probably helped each other through the loss, and now he knows she's being taken care of. He's right about that, but gets a nasty shock when he finds out they were actually together before he died. He ultimately chooses to forgive them, seeing that they're both remorseful and really are in love.
- In the episode "I Do", Ryder kissed Jake's girlfriend, Marley, despite being close friends with him. Marley would kiss him an episode later, and admit to Ryder that "[she] let him."
- Quinn gets pregnant after sleeping with her boyfriend Finn's best friend Puck, despite that she never slept with Finn.
- The inciting incident of Gossip Girl is that Nate cheated on his girlfriend Blair with her best friend Serena, starting a feud between the two of them.
- The Good Wife: A case of cheating before they became best friends, but when Alicia finds out that her now-best friend Kalinda was one of the (many) women her husband Peter slept with, she takes it just as hard and refuses to ever forgive Kalinda.
- Grey's Anatomy: Derek Shepard's ex-wife Addison sleeping with his best friend Mark Sloan and him walking in on them is the reason he leaves Boston for Seattle and works at Mercy West to begin with. While this obviously ruins his relationships with both, the trope is deconstructed as in time Derek is actually able to make peace with both Addison and Mark, taking responsibility for his own part in creating the stagnation in his marriage that led to Addison straying and re-building his friendship with Mark.
- Homeland: Nicholas Brody's wife, Jessica, gets into a relationship with his best friend, Mike, because they assumed Brody's dead - except that he's been held as a POW by al-Qaeda for 8 years. When Brody returns, Jessica and Mike end the relationship so that the former can be with her husband. Unfortunately, things get really complicated due to Brody's role as a sleeper agent for al-Qaeda, and then his mission for the CIA affects his marriage. It doesn't help that Brody ends up having an affair with CIA officer, Carrie Mathison. Eventually, Jessica goes back to Mike and she and Brody agree to end their marriage.
- House: In the episode "Fidelity", a patient delays her diagnosis of African trypanosomiasis (or sleeping sickness) due to the fact that she could only have contracted it through sex, and she didn't want to admit she was unfaithful to her husband. At the end of the episode, it is revealed she had slept with (and presumably gotten the illness from) her husband's best friend, who he had been seen jogging with at the beginning of the episode.
- Innocent (UK): David's murdered wife Tara was two-timing with his best friend Tom. When Tom accidentally injured her during sex, it was suspected to be a case of Domestic Abuse.
- It's Always Sunny in Philadelphia: This seems to be the situation in "Mac's Banging the Waitress," but turns out to be NOT Exactly What It Says on the Tin. Mac seems to be hooking up with the Waitress, the object of Charlie's affection. However, it's a subversion; The Waitress hates Charlie, and in the end, it turns out she was stringing Mac along to help her retrieve a sex tape Dennis made of her. The two never even have sex at all.
- Joan of Arcadia: Subverted. Joan only thinks that Adam and Judith are having an affair after catching them hugging. In reality, it was merely platonic.
- Life with Boys: Tess initially hated her best friend Allie's new boyfriend, Bobby — he was her long-standing wrestling rival and she thought he was a jerk. However, the trio went to the movie theater, and when Allie left, that was Tess's chance to get to know Bobby more. They shared a kiss. Allie caught them and ignored Tess's apology. But eventually, Allie accepts that she and Bobby have a lot less in common than he and Tess do, and they hug it out.
- The Mentalist: In Redwood, two girls are missing from their abandoned car: a bride-to-be and her best friend. The friend soon turns up dead, the bride later with no memory of what happened. Jane notes that the fiancé seems torn between the two girls, and when the bride recovers her memories, she remembers the friend telling her she and the fiancé had a fling. This made her need to pull over, allowing the killer, a Stalker with a Crush who didn't want her getting married to close in.
- In the Murder, She Wrote episode "Keep the Home Fries Burning," a few people in a restaurant are struck by what appears to be food poisoning, and one of them dies. Jessica realizes that the common denominator in the situation is a missing jar of strawberry preserves and that it only affected people who ate from the jar after it was at the table where the woman who died was sitting. It turns out that the poisoner had discovered her best friend and her husband were having an affair and had killed her friend in retaliation. She hadn't intended to make anyone else sick, but she hadn't been able to recover the jar of preserves fast enough to keep others from putting it on their toast.
- An episode of 9-1-1, has a woman and her best friend go to a "rage room" together so she can work off her frustrations. To get her in the mood, the friend brings up some of the things her husband has done to get her to break things. Then she brings up a sexual quirk that the woman never mentioned and the woman realizes he's been cheating on her with him. Now, the "friend" becomes the target of her rage and the 118 has to be called in.
- Orphan Black: Allison sleeps with her best friend's husband, Chad, in her minivan.
- Pretty Little Liars: After a Time Skip, Hanna and Caleb broke up and Caleb was now with Spencer. But Hanna and Caleb still had chemistry and ended up kissing behind Spencer's back.
- Rake: In season one, Scarlett, who is Barney's wife, sleeps with his best friend, Cleaver. Barney eventually forgives Scarlett, but their marriage didn't last long after that.
- Spartacus: Blood and Sand: A rather complicated, tragic and disturbing example occurs in the prequel involving Melitta and Gannicus. Melitta has a Perfectly Arranged Marriage to Oenomaus, who has a brotherly relationship with Gannicus; he and Melitta are also close friends. Then they're forced to have sex with each other to entertain a visitor, unbeknownst to Oenomaus. Afterwards, they find themselves developing an attraction to each other. They initially try to bury their feelings and avoid each other as neither of them want to betray Oenomaus (whom they also believe would kill them if he found out), but when Melitta learns Gannicus is to be sent away from the ludus, she visits him so they can have consensual sex. Unfortunately, this inadvertently leads to Melitta's death. Oenomaus remains oblivious as to what occurred between his wife and best friend until several years later in Vengeance, when Ashur reveals the truth to break him. This causes a rift between Oenomaus and Gannicus until the end of the season, where they finally reconcile.
- Two and a Half Men: Charlie and his Love Interest Chelsea were in a Relationship Revolving Door for the past couple of episodes, but him sleeping with her Best Friend Gail was the final nail in the coffin for their relationship and she breaks off with him for good.
- The Walking Dead: Shane is the best friend and former partner of Rick. He and Rick's wife Lori start sleeping together after Shane tells her that her husband died. Rick is still very much alive and Lori is quick to take him back when they're reunited; she's also not happy with Shane lying about his certainty over Rick's fate. Things are further complicated when Lori finds out she's pregnant and isn't certain who the father is. A jealous Shane becomes increasingly antagonistic towards Rick and obsessive over Lori, even trying to drunkenly rape her. This culminates in Shane trying to kill Rick, who is forced to kill him in self-defense. Rick later acknowledges he knows that Lori's baby was fathered by Shane, but he still raises her as if she were his own.
- Whiskey Cavalier: In the pilot episode, Will Chase's fiancée is revealed to be sleeping with his best friend, Ray. Naturally, this leads to a bit of tension between Ray and Will for a while.
- Will & Grace: When Rob and Ellen announce their engagement Grace tries to turn down Ellen's request that she be maid-of-honor. After the couple leave, Grace admits to Will that she slept with Rob just after she dumped her boyfriend and he'd temporarily split with Ellen. Will promises to keep it secret but Ellen accidentally walks in when he's teasing Grace. Ellen is furious with them both, but Rob eventually wins her over after they have sex in Will's bedroom.
- Yellowjackets: In the 1990s, Shauna was sleeping with her best friend, Jackie's, boyfriend Jeff, and got pregnant by him, which becomes a major issue when they crash in the Canadian wilderness.
- Brown Sugar: D is a victim of cuckolding on "Shit, Damn, Motherfucker":
''Why are you sleepin' with my womanWhy are you sleepin' with my womanThis comes as a total surpriseI just can't believe my eyesMy best friend and my wife
- Played for Drama in Gary Moore's song "Over the Hills and Far Away"(also covered by Nightwish). The male lead goes to prison for a robbery he didn't commit, being unwilling to use his alibi because he was in bed with his best friend's wife at the time the crime was committed.
- Jethro Tull: Invoked in "Locomotive Breath" as part of the troubles the "all-time loser" is facing.
His woman and his best friend / In bed and having fun
- In Kristen Kelly's song "Ex-Old Man", the narrator's ex-husband cheated on her with her best friend.
- Lesley Gore: From her song, "It's My Party (And I'll Cry If I Want To)," during Lesley Gore's birthday, her boyfriend Johnny and friend Judy both disappear from her birthday party at the same time. When they return they are now a couple, Johnny having never even bothered to officially break up with the birthday girl.
- Sparks have the song "Tits" where the narrator is drowning his sorrows with his friend Harry. He laments how distant his wife has been lately and suspects that she's seeing another man. In the final verse, he says what they both already know: Harry's the one sleeping with his wife.
- Voltaire: In "Stuck With You", a husband and wife are singing about how miserable they are, with the problems increasingly escalating. One verse is about the two cheating on each other. The husband slept with her sister, and the wife, well:
Wife: Well then let's propose a truceWhat's good for gander is good for gooseHusband: Hey, now that's different, you took to bedMy favorite dog, and all my friends!
- Arthurian Legend: Many works based on King Arthur focus on the plotline of Queen Guinevere cheating on Arthur with his closest knight, Sir Lancelot (which was a relatively late addition to the legends). Adaptations vary on how close Lancelot and Arthur or Guinevere and Arthur are. Some even make Arthur older to make audiences root more for Guinevere and Lancelot, or make Lancelot younger so it's a Mrs. Robinson situation. But many end with Arthur being legally obliged to burn Guinevere at the stake. In the course of rescuing her, Lancelot kills several of his fellow knights of the Round Table, making this betrayal the beginning of the end for Camelot.
- Greek Mythology: After discovering his wife Aphrodite was having an affair with his brother Ares, Hephaestus started up a relationship with Aglaia, one of the Graces and Aphrodite's friend. In versions where Hephaestus divorces Aphrodite, he and Aglaia even get married and have a much happier life together.
- Be More Chill: By Act 2, Jeremy is dating Brooke, the Innocent Beta Bitch to Alpha Bitch Chloe. During a Halloween party, Chloe gets drunk and tries to have sex with Jeremy because she's jealous that everybody likes Brooke better. Brooke walks in on the scene and becomes upset at both of them, though she forgives Chloe quickly the following day when Chloe claims it was all Jeremy's fault. She forgives Jeremy as well at the end of the play.
- Camelot: Adapted from T.H. White's novel, one scene shows Arthur hoping against hope that Lancelot will ride to Guinevere's rescue on the day of her scheduled execution. Downplayed, since Guinevere and Arthur were established as an Arranged Marriage.
- One event in BitLife has your best friend's partner come onto you. If you choose to hook up with them, your friend will likely become your enemy within the next year.
- The Paranormal Files series opens with the revelation that protagonist Rick is moving back to his father's old house after discovering his girlfriend (to whom he was planning to propose) in bed with his best friend.
- Throughout Syberia, Kate receives calls from her boyfriend and best friend, with it becoming increasingly transparent that the former is cheating on her with the latter. The two later come clean with the confession, blaming it on Kate's long absence from New York. By that time, however, Kate is so alienated from her past life, she just breaks up with him and continues on with her journey.
- The Witcher 3: Wild Hunt: Yennefer starts the game miffed with Geralt because he had a relationship with her Best Friend Triss in the previous games. Oddly, she only seems to be angry at Geralt about it, and never holds it against Triss, despite the fact Geralt had amnesia and Triss was the one that knowingly entered a relationship with him knowing Yennefer would be upset about it.
- The World Ends with You: Discussed and subverted. Ai has a crush on Makato, but after seeing her go out with her best friend Mina, she quickly comes to the conclusion that they're going out behind her back. Ai's anger eventually attracts Noise, worsening her emotional state until Neku and Shiki are able to sort it out. They reveal that they were trying to set Ai and Makato up for a date.
- Katawa Shoujo: In one point at Shizune's route, you can sleep with Misha, Shizune's friend, who sought you out because her feelings towards Shizune will go unrequited. Keep in mind, Hisao was already in a relationship with Shinuze, so accepting would be two-timing it. If you accept the route, the story goes straight to Shizune's Bad Ending, as Shizune ended up betrayed and lonely from your actions, implying that she knew about the infidelity and grew aloof, wondering if she was the problem.
- Melody: Sophia offers MC a relationship on the side if he’s been tutoring her and has formed a romantic relationship with the title character. If MC accepts, and Melody and Sophia have also been fooling around, it can lead to threesomes.
- In Rumbling Hearts, Takayuki's girlfriend, Haruka, had an accident at the beginning of the story, leaving her in a coma. When Haruka's best friend, Mitsuki, helps Takayuki through his PTSD from the incident, they began developing feelings for each other and start dating. Three years later, Haruka wakes up. Takayuki and Mitsuki choses not to tell her about their relationship, leading to drama ensuing.
- American Dad!: In "Merlot Down Dirty Shame", Roger takes Stan's place (by Stan's request) on a trip with Francine to a vineyard. While there, the two get drunk and kiss each other. The rest of the episode revolves around Roger, not wanting to risk his friendship with Stan, going to absurd lengths to keep the guilt-ridden Francine from telling him.
- BoJack Horseman: In "A Horse Walks Into a Rehab," teen rehab patient Jameson sneaks out to a party (with BoJack as an unwilling accomplice) to see her boyfriend Dathan so she can dump him before he dumps her, only to discover he's already hooked up with her friend McCaitlyn. They justify their behavior by explaining that Jameson is "a mess." BoJack tries to comfort Jameson and get her back to rehab, but she steals a car and goes back to her own home.
- Daria: Tom and Daria ended up kissing while he was dating Jane. While Jane is initially angry about this, she and Daria remain friends after taking some space apart.
- Human Resources (2022): Rochelle kisses Dante the Addiction Angel, despite knowing that her close friend Emmy has also been sleeping with him and is deeply obsessed with him. While she tells herself that she should stop, as it's not fair to Emmy, she pursues the relationship because she believes they're truly in love. This enrages Emmy so much that she turns into a giant Hate Worm while calling Rochelle out.
- Star vs. the Forces of Evil: In the episode Booth Buddies, Star and Marco share a kiss in a photo booth, while Star is still dating Tom, who had become friends with Marco at that point.
- Total Drama: After Courtney and Gwen start forming a friendship in the episode "I See London", Courtney's boyfriend Duncan comes back in the game. Later that day, Gwen and Duncan kiss while in the confessional together. Courtney finds out in the next episode and refers to Gwen as a "boyfriend kisser" for the rest of the season.