Accidental Marriage is a trope where two characters suddenly end up married. This can happen because they marry while drunk or under the influence of drugs or hormones, they might wake up in an Alternate Universe where they are married, Time Travel can be involved, there could be external influences (it might be a case of Forced Marriage), it could be a case of Aliens Made Them Do It and/or misunderstandings with alien cultures... Writers have found many reasons to invoke this trope.
The marriage then changes the relationship between the characters in a significant way; often it happens to characters who weren't romantically involved before. One or both characters may have had prior feelings for the other one and awkward misunderstandings may lead to a lot of pining. Sometimes the characters will have to pretend to actually be together.
A popular subtrope is "waking up married in Vegas" and dealing with the consequences - usually ending up being stuck married for a certain amount of time. This usually allows the pairing in question grow closer to the point where they want to stay married. In these cases, the characters are only in Las Vegas for a short time before moving on.
The Marrying Type by artsypolarbear  Fandom: The 100 - ClexaDate: 2016Length: 247KGenre: Married in Vegas
Clarke Griffin isn't the marrying type. Anything but that. Marriage, for her, is out of the question.
And yet, through some sick and confusing twist of fate, she finds herself with a wife that she cannot recall a thing about. Hell, she doesn't even know her name.
Alexandria Woods cannot be married. Not to anyone, and especially not to a woman.
And yet, she is. Neither of them know how or when or why, but they're married at least on paper. And what ensues after their first (or second) meeting is a beautifully messy and eventful love story.
Rebound by astolat  Fandom: American Idol - Kris/AdamDate: 2009
The strategy had not included getting drunk himself and losing all sense of timing and being the tawdry and expensive rebound sex.
What happens in Vegas... by redtoes  Fandom: Arrow - Oliver/FelicityDate: 2013Length: 48KGenre: Married in Vegas, Amnesia, Fake/Pretend Relationship
Oliver and Felicity wake up married in Las Vegas. But it's not what you think... It's memory loss
and fake relationships
and diamond heists and dealing with inquisitive journalists.
What happened in Vegas... by Jules_Ink  Fandom: Arrow - Oliver/FelicityDate: 2014–2015Length: 296kGenre: Married in Vegas
It's all fun and games until you wake up hung-over and married to a stranger. Five years ago Oliver Queen and Felicity Smoak made a drunken mistake that could never be corrected. After years on a hellish island he comes back as a man on a mission only to find out that what happens in Vegas doesn't always stay in Vegas. (S1 Rewrite)
Accidents Will Happen by melusina; podfic by argentumlupine  Fandom: Bandom - Pete/PatrickDate: 2015Length: 8kGenre: Didn't Know They Were Dating
When Pete and Patrick opened their bakery
, they accidentally submitted the domestic partnership form instead of the business partnership paperwork. They discover this when same sex marriage becomes legal and all registered domestic partnerships convert to marriages. Shenanigans ensue.
Think Once, Think Twice by Fahye  Fandom: Brooklyn Nine-Nine - Jake Peralta/Amy SantiagoDate: 2014–2015Length: 47kGenre: Married in Vegas
"First one to cry annulment covers the other's weekends for six months," Jake says instantly. (or, the one where amy santiago falls in love with and gets drunk vegas married to jake peralta, not necessarily in that order)
All Or Nothing by waitwhathuh  Fandom: Carmilla (web series) - Laura Hollis/Carmilla KarnsteinDate: 2017Length: 50kGenre: Married in Vegas
For Laura, running into her ex is mortifying enough. What's more mortifying is waking up after a crazy night in Las Vegas next to her ex. And, wait, why are they both suddenly wearing wedding bands?
Ill Equipped by ghostwriterly  Fandom: Check Please! - ZimbitsDate: 2017Length: 72kGenre: Married in Vegas
Jack's first mistake was getting drunk in a salsa club with Kent Parson.
Don't Blame Me (It Was All a Blur Last Night)] by nerakrose  Fandom: Harry Potter - Draco Malfoy/Harry PotterDate: 2015Length: 19k
"Stop moving, Potter," Malfoy mumbled from where his head was buried under a pillow. "You're disturbing my hangover. Also, why are you still here?"
"This is my hotel room," Harry told him.
Never Say Never Fandom: MCU - Steve Rogers/Tony StarkDate: 2012Length: 37k
When 24-year old Tony Stark goes out to distract himself from the fact that he has been put on probation by his company's board of directors due to his partying lifestyle, he bumps into (quite literally) a law student named Steve Rogers, who has recently lost his father and has been 'dragged out' by his best friend to "have some fun." "Some fun" turns into shots, Vegas, and...marriage?
Turn Off Trouble Like You Turn Off a Light by blithers  Fandom: MCU - Darcy/SteveDate: 2012–2013Length: 31kGenre: Married in Vegas, Road Trip
Oh, God,' he said, and she knew just enough about Captain America to find this a pretty strongly worded statement on the situation. (Or, Steve and Darcy wake up married in Vegas.)
Sixpence In His Shoe by scifigrl47  Fandom: MCU - Steve Rogers/Tony StarkDate: 2017Length: 103k
Steve and Tony should really read the fine print on what they're signing. Then again, some mistakes are not really mistakes.
Truly, Madly, Crazy, Deeply by harriet_vane  Fandom: One Direction - Liam/LouisDate: 2012Length: 30kGenre: Married in Vegas
A non-famous AU
in which Liam and Louis wake up married after Zayn's bachelor party in Las Vegas.
nevada's fault by irishmizzy and miss_bennie  Fandom: One Direction - Niall/ZaynDate: 2014Length: 34kGenre: Married in Vegas
Tourfic in which Niall and Zayn drunkenly get married during a game of Truth or Dare
after a Las Vegas concert. Not an AU.
(wise men say) only fools rush in by singsongsung & sylwrites  Fandom: Riverdale - Betty Cooper/Jughead JonesDate: 2017–2018Length: 49kGenre: Married in Vegas
He walks over to her and surprises her by reaching for her left hand. He lifts it up, and she’s startled to see that she’s wearing two diamond rings. She stares at them, her hungover brain not fully understanding their significance - and then Jughead lifts his left hand to show her a silver ring on his own fourth finger.
Never Marry for Money (You Can Borrow it Cheaper) by hapakitsune; podfic by knight_tracer  Fandom: The Social Network - Mark/EduardoDate: 2011Length: 71KGenre: Fake/Pretend Relationship
Eduardo & Mark have to attend some kind of shareholder's/young-and-a-billionaire's meeting in Vegas/Vermont/Illinois. One thing leads to another and they WAKE UP MARRIED. A quicky divorce/annulment would be easy if not for the fact that Mark drunkenly changed his facebook status to MARRIED TO EDUARDO SAVERIN.
Or, Mark and Eduardo get drunkenly married. Chaos ensues.
Of Convenience by annenburg  Fandom: Star Trek: AOS - Kirk/SpockDate: 2009Genre: Fake/Pretend Relationship
Kirk needs to get married for political reasons. Spock offers himself as the logical choice.
The Morning After by noodleinabarrel  Fandom: Star Trek: AOS - Kirk/SpockDate: 2016Length: 50kGenre: Accidental Bonding
Jim convinces Spock to take shore leave with him on Risa, hoping the time together will help re-solidify their bond of friendship after some recent tension. Meanwhile, Spock convinces himself he's on Risa for one reason and one reason only, to prevent his wayward captain from getting into trouble.
After a passionately illogical night of Romulan Ale and chocolate infused liquor, everything changes when Jim wakes with something other than a hangover filling his head. Something he's sure neither he nor Spock can handle. Because if Jim knows anything for sure, it's that his messed up thoughts belong nowhere near Spock's clean, ordered mind.
step out into the sun by plutos  Fandom: Star Wars - Poe Dameron/FinnDate: 2016Length: 71kGenre: Fake/Pretend Relationship, Pining
Otherwise known as: Finn Is An Oblivious Numbskull, or: the one where Finn gets a name, a jacket, a droid, and a marriage out of nowhere in only a few short weeks
A Strong Heart and a Nerve of Steel by lupinus and uraneia  Fandom: Teen Wolf - Derek/StilesDate: 2013Length: 21kGenre: Fake/Pretend Relationship, Married in Vegas,
Stiles and Derek wake up married in Vegas. Well, they would have if it was legal. In which Stiles is the president's son, Derek is his bodyguard, and Papa President orders them to pretend to be in love for the sake of gay rights.
Impulse Decisions by listerinezero  Fandom: XMFC - Charles/ErikDate: 2012Length: 41kGenre: Married in Vegas, Modern AU
Erik wakes up in Las Vegas with a hell of a hangover, a telepath in his bed, and a ring on his finger. Now what?