David: Sleeping with my wife.
Whale: Look, I didn't know, alright? I was cursed.
David: Yeah. I got it. What do you want?
When an adulterer tells their wronged partner that they never meant for this to happen, the stock response is, "What, you tripped and fell out of your clothes and into bed together?" While this is obviously facetious, there are circumstances in which it is possible to cheat on a committed partner by accident.
The unwitting adulterer may have been the victim of a Bed Trick and believed it was said partner with whom they were sleeping. They may have come down with a nasty bout of Easy Amnesia and forgotten they ever had a partner to whom they owed fidelity. A tragic miscommunication may have caused them to believe their partner had broken up with them or given them permission to take an additional lover. Or maybe they Thought You Were Dead, went through a decent, respectable mourning period (or sometimes not), and moved on to a Second Love or comforting fling.
The "Reports of My Death Were Greatly Exaggerated" version can be a way to add a little extra heft to a Love Triangle, play with any number of Adultery Tropes while keeping all involved parties sympathetic, or add even more pathos to a character who is a victim of war, shipwreck, coma, Easy Amnesia, or any number of other already-tragic circumstances. Alternatively, if the widow/er/ed party finds a new Love Interest almost immediately after the first is presumed dead, it can call their loyalty into question.
Reactions from the adulter-ee vary greatly depending on their personality and the circumstances. Some will feel just as aggrieved as if their partner had intentionally betrayed them ("Did you even try to look for me?"). Some will be at least somewhat understanding, but still hurt and confused about where this leaves them going forward ("But what about us?"). A few will be completely understanding and either brush off the accidental infidelity as "nothing to forgive" or even step aside for a Second Love, commonly in circumstances where they were believed dead and there could have been no reasonable expectation they would ever turn up alive, such as if they actually did die.
The person the Accidental Adulterer "cheated" with tends to experience a similar range of reactions, depending on whether they knew of the circumstances or not. Villainous examples who intentionally hid the truth from the Accidental Adulterer in order to get in their pants may attempt to finish the job by Murdering the Hypotenuse. Sympathetic examples will be apologetic and may even step aside.
Accidental Adulterers themselves tend to be troubled and confused more than anything, but will usually settle on continuing their relationship with one Love Interest or the other, assuming a convenient (real) Death of the Hypotenuse doesn't make the choice for them. Occasionally, they will find it impossible to choose and attempt to hang on to them both.
In Real Life, one reason for the legal concept of Legally Dead is so that people who reasonably believe their spouse to be dead but who cannot positively confirm it can remarry. (The other reason, of course, being so that heirs can get their inheritance.) Naturally, this doesn't tend to make the situation any easier to deal with when this happens.
- Shinkon Gattai Godannar!!: Mira Ackerman, who was in a relationship with Goh, has been presumed dead for 5 years, but in reality has been in a coma. By the time Goh finds out, he has already been married to Anna.
- In Octave Setsuko tells Yukino that it is okay for her to date men. Yukino takes this as Setsuko giving her the okay and has a one-night stand. It turns out Setsuko meant "If we weren't dating..." and is angry.
- In The Walking Dead, main character Rick's wife Lori sleeps with best friend Shane when Rick is presumed dead in the Zombie Apocalypse. It plays more like standard adultery than most examples of this trope, since it had only been a few days or weeks and they have no evidence for Rick's death beyond, "yo, zombies." When Rick returns, Lori considers it a mistake and wants to forget it ever happened, but Shane has other ideas.
- In Ultimate Marvel, Bucky marries Captain America's fiancée during the latter's long stint as a Human Popsicle. Considering that Cap emerges still physically in his twenties to find Bucky and his former fiancée as senior citizens, a resumed romance was unlikely in any case.
- In Pearl Harbor, Rafe is presumed dead after being shot down over Nazi Germany. His Love Interest Evelyn begins a romance with his best friend Danny, which makes things awkward when Rafe returns alive. Fortunately, Danny has the good manners to get himself actually killed on their next mission, leaving both of them Someone to Remember Him By.
- In Cast Away, Chuck's fiancée Kelly says that she "always knew" he was alive, but felt she had to let him go, and married a dentist. She offers to divorce her new husband to get back together with Chuck, but he doesn't want to break up her new family.
- Rick and Ilsa of Casablanca have this sort of tryst after Ilsa's husband Victor is presumed dead. She leaves Rick after Victor turns up alive, but much of the film is concerned with her being torn between the two of them.
- The main character of Random Harvest is an amnesiac who, oblivious to the fact that he is already married, becomes engaged to a young admirer. His wife tracks him down and attempts to trigger his memories of her; his new fiancée breaks off the engagement before this trope can take full effect.
- Inverted in the backstory. Dante's girlfriend Caitlin once intended to cheat on him at a costume party and arranged to meet another guy in a darkened bedroom. She got the wrong bedroom and instead unknowingly had sex with Dante, who was there passed out.
- Played straight later in the film. Shortly after getting back together with Dante, Caitlin enters a darkened bathroom and has sex with what she thinks is an uncharacteristically silent and stoic Dante. It turns out to be the corpse of a man who had earlier suffered a fatal heart attack. Upon learning the truth, she goes into shock.
- A double-example in Six Days Seven Nights. When Quinn and Robin's plane goes down over the ocean, everyone fears the worst. Their respective lovers fall into each other's arms for comfort, even though the search was still technically ongoing. To Frank's credit, he tried to resist, and felt terrible about it, even before he found out Robin was alive. By the time he does, Robin and Quinn have fallen in love anyway, so everything more or less works out.
- Averted in The Odyssey; presumed-widow Penelope has no shortage of suitors— some quite forceful— while presumed-dead Odysseus is Lost at Sea, but she stays faithful for ten years. More faithful than Odysseus himself, for that matter, although "he never gave consent in his heart" and saying 'no' to goddesses was a good way of making sure he never got home.
- The Lost Fleet: Towards the middle of the original series, Rione, who has been sleeping with Captain Geary up to this point, discovers that her husband might actually be alive and kept as a prisoner of war. This causes her to immediately break off her current relationship, which was already a Friends with Benefits situation at best. A few volumes later said husband actually turns up, and... Well, suffice to say he didn't take it very well when he found out. In the end, though, Rione chooses to die at her husband's side, whose mind has already been warped by the secret government experiments, while saving everyone else in the process.
- Animorphs: Marco's mother Eva is thought to have died in a boating accident, but in actuality she was infested by the parasitic alien Visser One, who staged the accident as a cover. Marco eventually discovers this, but is forced to keep it and his double life a secret. However, his dad starts courting another lady Nora, and Marco has trouble accepting her knowing that his real mother is still alive. He ultimately lets them marry, deciding it's better that they move on, but the battle with the Yeerks continue to complicate things. After a while Nora is infested and Eva freed, but she and Marco's dad have some trouble reconnecting, since she's been changed by her years enslaved and he's long moved past her "death".
- In Harry Turtledove's Worldwar books, a nuclear physicist named Jens Larssen goes on a cross-country trip through the war-torn US to deliver an important message and is not heard from for several months. Everyone, including his wife Barbara, assumes he's been killed, and she eventually ends up having Glad-to-Be-Alive Sex with Sam Yeager (they both just survived being strafed by a lizard killercraft). They then start sleeping together and, eventually, get married. Meanwhile, Jens is desperately trying to reunite with her and send her a message, with the US Army preventing him from doing the latter, as his work on the Manhattan Project is considered to be too important. By the time Jens and Barbara are reunited, she is already Barbara Larssen Yeager and is carrying Sam's child. She refuses to leave Sam, despite the fact that Jens being alive technically invalidates her second marriage. It's even made clear that the biggest reason why she stays with Sam is because she's pregnant. Jens then resolves the Love Triangle by going off the deep end, killing a US soldier, and trying to defect to the lizards with his knowledge and getting killed for his trouble. On Jens's side, he does end up cheating with Barbara with a woman he meets on the way, who also gives him the clap.
- In The Scarlet Letter, Hester is separated from her husband, and believes him to be dead. While she's grieving, she's comforted by Rev. Dimmesdale, in more ways than one. She becomes pregnant by him, which is a big taboo in their Puritan society. To add insult to injury, she finds out that her husband was not dead. She is estranged from him for the rest of the story, though her husband is understanding, more concerned with finding out who her child's father is than trying to punish her. It's really Dimsdale he wants revenge on.
- In the Sherlock Holmes short story "The Adventure of the Noble Bachelor", the titular character's young bride disappears because her husband, whom she had given up for dead, turned up alive at the wedding.
- In Legends of Dune, Serena Butler is engaged to Xavier Harkonnen during humanity's desperate struggle against the Thinking Machines. During a trip to another planet, her ship is captured by the machines, who take her to Earth, where independent robot Erasmus expresses an interest in her fierce personality, wishing to study her further. Back on Salusa Secundus, everyone assumes Serena is dead, and the grief-stricken Xavier ends up finding comfort in the arms of Serena's sister Octa (who has long had a crush on her big sister's fiancé), eventually marrying her and having several children. Serena turns out to be pregnant by Xavier and gives birth. After a while, Erasmus gets fed up with the baby's crying and constant need for attention from his research subject, so he casually throws the child off a high balcony, inadvertently inspiring the human slaves on Earth to revolt, kickstarting the Butlerian Jihad. After Serena returns to Salusa Secundus, she is deeply hurt by finding out that Xavier has moved on, but understands why he thought she was dead. This is one of the reasons why she ends devoting herself to the Butlerian Jihad and willingly sacrificing herself in order to inspire the people to destroy the Thinking Machines.
- In a variant, We Can't Rewind has a case of accidental fornication due to a "Freaky Friday" Flip leading to certain erroneous assumptions concerning identities. The step-siblings Jackie and Jaymee assume that being in their married parents' bodies means they are their parents, and therefore they are married. Upon discovering this, their parents immediately scold them and set them straight.
- In Star Trek: Deep Space Nine, Odo gets involved with an informant he's protecting. Turns out, she's an undercover agent for a government that uses Easy Amnesia and Fake Memories to ensure their agents believe their own covers, making them virtually impossible to detect. When her mission is complete, her original memories are restored, and it turns out she's married. She takes it in stride, having known the risks of the mission. Odo seems to take it much harder.
- At the start of Star Trek: Voyager, Captain Janeway is engaged to a civilian named Mark Johnson when the titular ship becomes stranded on the other side of the galaxy. Mark eventually marries someone else, which Janeway finds out when Starfleet reestablishes contact with Voyager in "Hunters", 14 months after the ship went missing in action and she was declared Legally Dead. What really stings about this is that she'd still been holding out for him all this time, passing up a possible romance with her first officer Chakotay and an offer from Q to transport her ship home in exchange for having a baby with him, among other things.
- Karen on Will & Grace starts up a relationship with another man after her husband Stan supposedly dies. Stan later turns out to have faked his death, but Karen has already divorced her new husband by this time. (In fact, she requests a divorce during the wedding reception.)
- One case on Night Court concerns a Vietnam veteran who went MIA and eventually came home to find his wife remarried. This proves to be a hard choice for the wife because she loves both of her husbands. Later, husband #1 saves husband #2 from choking and decides to move on.
- Played with on Cheers. Cliff gets bitten by a dog on his route and decides to sue the owner. The owner then starts up a relationship with him instead. Eventually, she gets him to sign a document dropping the lawsuit and checks her messages. One of them is from her husband who disappeared in Vietnam and just came back. (It's heavily implied to have been a huge con and Cliff fell for it.)
- Friends has a famous example of the Poor Communication Kills variety; during an argument, Rachel tells Ross that they should "take a break... from us". Ross becomes upset and storms out. Rachel's co-worker Mark comes over to help her through the fight, and she makes clear that she doesn't want to break up, but when Ross calls to talk to her, Mark answers the phone. Ross takes this as confirmation that they're broken up, gets drunk, and has a One Night Stand. Rachel considers it cheating; Ross repeatedly and vociferously disagrees. Who is right is a matter of some disagreement among the base.
- Occurs in season 1 of Once Upon a Time. Most of the cast are fairy tale and storybook characters who were stripped of their memories. Snow White is married to Prince Charming, but their Storybrooke counterparts don't remember each other. Snow has a one-night stand with Dr. Whale/ Dr. Frankenstein, while Charming sleeps with Katheryn/Princess Abigail (King Midas's daughter) thinking she's his wife.
- A twist on the "Reports of My Death Were Greatly Exaggerated" version occurs near the end of season 3; Robin's blossoming relationship with Regina is shattered as a result of his wife Marian, who had been dead, appearing alive and well after Emma accidentally goes back in time, saves her life, and brings her forward to the present. It turns out to be a villainous example, as "Marian" is really Zelena in disguise; Zelena killed the real Marian while Emma's back was turned and switched places with her. Robin ends up sleeping with Zelena thinking she's Marian and feeling bad about his not-actually-an-affair with Regina. He does eventually discover the truth and gets back together with Regina. Then in season 5 Zelena tricks Robin into kissing her by disguising herself as Regina.
- Downplayed: in Season 5, while in the underworld, Mary Margaret accidentally kisses David's twin brother James on the lips, thinking he is David.
- In Eureka, Allison ends up accidentally getting launch along with the actual crew of the Astraeus. They finds themselves back at Eureka but are told that 4 years have passed since their departure. To her horror, Allison finds out that Jack is now with Jo, having thought her dead, and has no intention of getting back with her. This turns out to be not true, as the crew is actually stuck in a virtual reality world created by the Consortium.
- The Young and the Restless: Nick comes home drunk after partying with friends to celebrate his college graduation. Meanwhile, a friend of his wife's is staying over to help her look after the baby, as she isn't feeling well. Nick stumbles into the nursery (apparently so drunk he doesn't realize it isn't his bedroom, finds the friend in bed, and thinking that she's his wife (aside from his intoxication, it's dark and the two women resemble each other) makes love to her.
- The Bold and the Beautiful: Ridge Forrester sleeps with ex-girlfriend Morgan, who has asked him to father her baby, believing that he has permission from wife Taylor to do so. It turns out that Morgan faked the permissive e-mail that Taylor supposedly sent.
- Also, during a masked ball, Brooke thinks she has enticed Ridge into some Wall Bang Her sex. It turns out that the man in question was her daughter's boyfriend, making her play this trope twice.
- All My Children. Doped up on pain medication, Erica throws herself at her doctor. Not until she calls him by her husband's name do either of them realize what's happened.
- Sunset Beach. Ben's evil twin Derek takes his place in his life and soon seduces his fiancee Meg, basically raping her as she believes it's Ben she's making love to.
- A "Reports of My Death Were Greatly Exaggerated" version occurs on American Horror Story: Asylum. Kit Walker, committed to the criminally insane asylum for the supposed murder of his wife Alma, falls for another inmate, Grace Bertrand, and has a romantic and sexual relationship with her. Upon his release, he takes Grace home, to discover Alma alive and well; she had been abducted by aliens and returned years later..
- In The Who's Tommy, Tommy's father returns from war years after going missing and discovers that his wife has taken a lover, now Tommy's stepfather. It doesn't go well for the lover (or Tommy's father in the film version).
- The French song "Brave Marin" is about a sailor who goes into a bar and starts drinking and singing, while the bar's owner starts crying. He asks her why, she says he reminds him of her dead husband. He then notes how many more children she has (it can be anywhere from three to twelve depending on the version). She's received so many letters saying he was dead that she remarried. The sailor finishes his glass and goes back to his ship.
- Funky Winkerbean: Happened to Wally Winkerbean twice, both times due to being taken prisoner during the War On Terror and being listed as missing in action and presumed dead.
- 9 Chickweed Lane: Although they are not actually married, Edna is engaged to Bill before he goes missing, presumed dead, during the war. Many years later she ends up with Kiesl, just in time for Bill to show back up alive.
- Depending on the Writer; in versions of Sweeney Todd The Demon Barber Of Fleet Street in which Todd and Mrs. Lovett's relationship is romantic, such as the Stephen Sondheim musical and the Tim Burton film version, it comes out that Todd has unwittingly committed adultery when his wife Lucy turns up alive but insane. He is not happy with Lovett when he finds out that she knew all along.
- In Catherine the protagonist Vincent wakes up from a night out with his friends with a younger blond woman named Catherine in his bed. He has no memory of how she got there, but he knows his girlfriend Katherine won't be happy. It turns out that she was a succubus sent by Thomas Mutton, AKA the Dumuzid, to seduce him and many other males, forcing them to fall into the Trials (ironically, however, she isn't actually sleeping with most of them, Vincent included). Depending on the ending, Vincent either manages to patch things up with Katherine due to the fact that the nature of the encounters (Catherine never appears to anyone else other than who she's currently seducing, meaning the whole thing comes off as made-up) being explain as symptoms of stress, or he ends up becoming an Incubus and taking over hell with Catherine.
- The Adventures of Dr. McNinja: Defied. Mitzi McNinja only realized that her husband faked his death because her suitors kept turning up dead.
- Dangerously Chloe: time travelling succubus Chloe, in need of a quick distraction, zaps Teddy's father and the women she assumes is Teddy's mother with a lust spell. Unfortunately she wasn't Teddy's mother, she was his maternal aunt and the resulting affair destroyed his parents' marriage.
- Zig-Zagged in the Family Guy episode "Perfect Castaway"; when Peter returns from being Lost at Sea, he finds that Lois has remarried with Brian. Yes, the family dog. However, much to Brian's frustration, they have never actually consummated.
- Inversion: Bender attempts to prove that his girlfriend Anglelyne still has feelings for her ex-husband Flexo by disguising himself as the latter and attempting to seduce her. When she responds to "Flexo's" overtures, Bender reveals the ruse and accuses her of disloyalty. She responds that "Maybe I love you so much that I love you no matter who you're pretending to be!" Given that what won her over was "Flexo" acting more like Bender than like the actual Flexo, she may have a point.
- Also happens in the second movie when Kif is killed, and while he gets better he finds out that Amy in her grief slept with Zapp Brannigan, who sketchily took advantage of the situation.
- In "The Butterjunk Effect ", Fry accidentally sleeps with Amy, thinking she is Leela, because he didn't turn the lights on.
- Played for Laughs in the South Park episode "Prehistoric Ice Man," where the titular character, Larry, was a Human Popsicle for 32 months. Even though they searched for him all afternoon, there was no sign of him, and though his wife waited three whole days, she quickly realized that she had to move on. Not only is she already remarried, but she barely remembers having been married to Larry at all, and she and her new husband have kids who are eight- and ten-years-old.
- In Spawn, upon coming back as the titular character, Al Simmons discovers that his wife has since remarried with his best friend. He is initially enraged at both of their lack of loyalty, until he regains part of his memory and realizes that, knowing that his was a dangerous profession, he actually asked his friend to take care of his wife should anything happen to him.
- A non-sexual version in Star vs. the Forces of Evil. Marco and Star share a kiss after the former admits to reciprocating the crush that Star told him she had several months prior. The problem? Star is currently dating Tom, who views Marco was his best friend to the point that, when Marco does tell him about the incident, he assumes it was a bluff in order to show his loyalty to their friendship. Even after clarifying, Tom is disappointed but still amicable (granted, they had more important issues than a love triangle to deal with at the time).