So, you're writing a story and you need your character to be both immediately sympathetic to the audience and motivated to suddenly change their life in a dramatic way: How about you have them walk in on their partner in the middle of having sex with someone else? Witnessing this infidelity shows that the protagonist as down on their luck and implies that their personal life was unhappy and/or unfulfilling, paving the way for the drastic change that comprises the work's gimmick. It also establishes them as single, just in time for the introduction of the work's Love Interest.
Works that use this trope rarely focus on the ex or the causes of the affair (if they do, the ex is rarely portrayed as a Sympathetic Adulterer, since the audience needs to root for the protagonist, and may become a villain with this trope as an Establishing Character Moment), and generally do not try to salvage the previous relationship after it has served its purpose.
- Sliding Doors: While the two timelines split with Helen missing the titular doors, the more important catalyst for divergence is when the Helen who caught the train walks in on her boyfriend cheating on her.
- Mr. Right: Martha begins the film by breaking up with her boyfriend after catching him cheating on her. This conveniently leaves her free to start dating ex-hitman Francis.
- Head Over Heels: Amanda walking in on her boyfriend cheating on her is what leads her to moving in with her friends, and eventually, start spying on her neighbour.
- Something To Talk About: Grace finds out her husband is cheating on her, which causes her to move into her parent's ranch.
- Love Actually: Jamie (Colin Firth), a dissatisfied writer, walks in on his girlfriend cheating on him with his brother. This makes him go to France to write for some time, where he meets his love interest Aurelia.
- Old School opens with Luke Wilson's character coming home early from a business trip to find his girlfriend watching pornography. He initially thinks this is actually a turn-on and gets excited... until several naked people enter the room and her cheating becomes apparent. And then he goes and moves out and begins a frat for old people.
- Lucky Number Slevin: Slevin is staying at Nick's place because he walked in on his wife cheating on him. This leads him to being mistaken for Nick and asked to Work Off the Debt that Nick had with a mob boss. Subverted by the fact that it turns out Slevin was lying about this story.
- This Is Where I Leave You: Judd's (Jason Bateman) misfortunes begin with him finding out his wife is cheating on him... with his boss, leading him to lose his job.
- In Nothing to Lose, Nick comes home and finds his boss's cufflinks on the kitchen counter. He hears a woman moaning upstairs, so he walks up and spies his wife riding a man. Devastated, he leaves, goes for a drive, and a carjacker named T-Paul tries to rob him at gunpoint. Not caring what happens to him anymore, Nick puts the pedal to the metal, ignoring T-Paul's gun in his face. Later on, when they resolve their differences and become friends, Nick decides to rob his boss, since, as a loyal assistant, he knows the boss's safe code. He later confronts his wife about the adultery, and finds out that her sister is visiting them with her husband. Apparently, they decided to take advantage of the owners not being home and screwed in their bed (and Nick only saw the woman from the back). The cufflinks were left by his boss the last time they invited him for dinner.
- In Bad Moms, Amy walks in on her husband having videochat sex with a woman from halfway across the country. She kicks him out of the house. This is just one in the long line of straws that finally cause her to snap. They do try counseling at one point, but even the therapist admits that they're probably better off on their own. This frees her up for a very attractive widowed dad at her kids' school.
- Me Myself And I: The series follows the events of Alex's life following three separate life-defining incidents that occurred in 1991, 2017 and 2042. The middle storyline follows Alex's life after he walked in on his wife sleeping with a paramedic.
- The Orville: The series opens with Ed walking in on his wife Kelly having sex with an alien. The subsequent breakdown is why his career is so far off track for the rest of the pilot. It is revealed at the end of the pilot that the entire series can be traced to this event, as it is Kelly's guilt that led her to pull the strings to get Ed his post as Captain of the Orville. Unlike a typical example of the trope, Kelly does, in a way, appear as a Sympathetic Adulterer, and even Ed admits later that him working too hard and ignoring her probably played a part in it, although he doesn't absolve Kelly of the responsibility (and neither does she). He even thanks her at one point (albeit in a slightly sarcastic manner) for cheating on him, thus sparing them both a marriage that was clearly not working. A woman Ed hooks up with does ask him why his wife didn't simply file for divorce, if she wasn't happy with him. Later on, the alien Kelly slept with shows up aboard the ship, and it's eventually revealed that his species secretes pheromones once a year, with effects very similar to a date rape drug. Both Kelly and Ed end up sleeping with him because of that (not at the same time), while Dr. Finn ends up doing... something with Yaphit, whom she normally wouldn't even date. The ending leaves it ambiguous whether the original infidelity was willful on Kelly's part or under the influence of the pheromones.
- New Girl: The series begins with Jessica Day coming home early to surprise her boyfriend while wearing nothing but a trenchcoat (and a ribbon). Unfortunately for her, he's cheating on her with another girl and she winds up moving out, leading to the Roommate Com premise of the show.
- She isn't the protagonist, but Jane the Virgin's convoluted plot is kickstarted when Luisa walks in on her wife cheating on her. The fragile state of mind this puts her in leads to Luisa making the crucial mistake of accidentally artificially inseminating Jane instead of Petra. Luisa herself takes the split badly - she returns to her former alcoholism, goes soul-searching in some temples abroad, and hooks up with her ex-slash-stepmom.
- Crashing: Semi-autobiographical, this series stars Pete Holmes As Himself. After walking in on his wife (played by Lauren Lapkus) cheating on him, Pete decides to try his hand at Stand Up comedy and must crash on the couch of his comedian friends as he no longer has anywhere to live.
- Necessary Roughness: Dani finds out that her husband has been having affairs right under her nose. She promptly kicks him out of the house, files for divorce, and gets a job as a therapist for a professional football team.
- Already disillusioned with her glitzy plastic surgery practice in Los Angeles, Dr. Sydney Hansen decides to return to her family in Providence after walking in on her boyfriend in the shower with another man.
- The Soap Opera Sunset Beach kicks off with the heroine Meg pulling a Runaway Bride on her fiance after finding out that he's been sleeping with her maid of honor. . .to flee to the titular town to meet the man that she's been having an online affair with.
- The main plot of Life Is Strange: Before the Storm is kicked off by Rachel pulling Chloe into her investigation of a secret her father appears to be keeping, which turns out to be an affair with a mysterious woman.
- Referenced in an episode of Cracked's Obsessive Pop Culture Disorder. In the episode "6 Bizarrely Specific Scenes Hollywood Won't Quit Using, this trope is covered and examples from several works are given and reasoning behind it is explored. The basic conclusion is that it's often used as an excuse for the (almost invariably male) lead to leave his old life behind and start anew.