The guests have arrived, the priest is waiting at the altar, and the organist is playing "Here Comes the Bride". There's only one problem: the happy couple isn't getting married after all. A resourceful character suggests that (a) another couple get married in their stead (alternately, an older married couple may choose to renew their vows), (b) the original bride (or, less frequently, groom) marry her (or his) true love, or (c) everyone just have a big party. The important thing is that the wedding paraphernalia won't go to waste.
If they already got most of the way through the wedding the first time before it got called off, somebody may suggest they Skip to the End on the do-over.
In the Post-Zero HourLegion of Super-Heroes, Saturn Girl and Cosmic Boy's marriage gets called off when it's discovered he's currently comatose, and she's been subconsciously manipulating his body telepathically for... a while.. While pretty much everyone else is busy dealing with that, Ultra Boy and Apparition take advantage of the minister being present to get married instead.
In The White Wolf, the prince's wife had reached him before he married another bride. He marries off his new bride to one of the guests.
In Princess Belle-Etoile, the king had resolved to marry again, but his own children appeared at the wedding feast and told how he had been deceived about his first wife. He takes her back, and rather than waste the feast, marries his daughter off to her cousin.
Mamma Mia!: When Sophie and Skye decide to delay their wedding to travel the world, Sam reveals that he's always loved Donna, is now divorced, and proposes to Donna, using the trope name.
The Princess Diaries 2: Royal Engagement ends with Mia deciding she'll become Queen without having a husband and gets the marriage law abolished while respectfully letting her groom off the hook. Instead, her grandmother Clarisse and her bodyguard Joe get married.
LaWanda in the movie Problem Child 2 is Ben Healy's fiancée, but she decides to marry Ben's father instead when the wedding is crashed.
Spaceballs starts with Princess Vespa ditching her wedding to Prince Valium. Unfortunately, since he's the only prince in that part of the galaxy, she still has to marry him, so at the end of the film they try to marry a second time. At which point Lone Starr bursts into the chapel, revealing his newly discovered heritage as a prince. Thus Vespa and Lone Starr get married instead. Valium, being a complete narcoleptic, doesn't even notice.
This was relatively common in pre-90s Bollywood films; usually the rationale was that the local astrologer had gone to a lot of trouble to find the most auspicious possible date, and postponing the wedding could be even more disastrous than playing eeny-meeny-miney-moe in search of a replacement groom.
In Madea's Family Reunion, near the end, Lisa decides to cancel her wedding to Carlos after he abused her. The wedding planner laments that her lavish Paris-themed wedding will not be used. Lisa's sister Vanessa's boyfriend decides to propose to her right then. She accepts and they get married.
In Runaway Bride the decision (same bride, different groom) is made the night before, but the trope still fits. Naturally, she ends up running.
In Corpse Bride, this trope happens to Victoria while Victor's trapped in the Land of the Dead with Emily. Upon learning Barkis is unmarried, the Everglots promptly decide to substitute him in for the groom (unaware, of course, that he plans to kill Victoria in hopes of stealing money they don't have.) Mayhew, upon relaying the news to Victor, even jokes that they "didn't want to waste the cake."
And a second time at the very end, when Emily is freed of her curse and lets Victor marry Victoria, in this case the wedding initially was to be Victor and Emily's.
Foolin' Around: when Annette O'Toole decides to marry the protagonist (Gary Busey), her mother (Chloris Leachman) 'saves face' by marrying her butler (Tony Randall).
Implied in Shrek, since the final two scenes skip from Fiona and Farquaad's ruined wedding to her and Shrek leaving on their honeymoon.
Nanny McPhee at the climax of the film as Cedric and the children ultimately wreck the wedding to the wretched Mrs. Quickly...but Cedric still needs a wife to meet his former aunt-in-law's deadline to continue her much needed financial support. However, he and Evangeline decide to marry on the spot, thus satisfying the requirement set for Cedric (though he also has to quickly explain to Adelaide that Evangeline wasn't actually his daughter).
At the very end of In And Out, after Howard and Emily's wedding is called off, the venue is still used for a wedding and reception anyway - by Howard's parents, who renew their wedding vows.
A rare example of the C-type, in The Movie adaptation of The Beverly Hillbillies, the con artists are thwarted at the end of the film and Jed's wedding to the female con artist is off. They just decide to throw a big party instead rather than waste the wedding.
In Once Upon a Marigold, the princess' Arranged Marriage gets called off once the evil queen is gone, but the princess' true love has just conveniently shown up and she marries him instead.
Semi-subverted by Thursday Next, because even in her admittedly screwy reality you need to formally advertise a marriage in advance; however, they immediately book the same church for a month hence, and the festivities are repurposed as an engagement party.
Possibly played straight in The Westing Game. While it's uncertain how early Angela decides against marrying Doctor Deere, everyone still holds a wedding for Crow and Otis Amber instead.
Dave Barry mentions a friend of his who was in the mood to have a wedding, but all but one of her children were already taken. So she decided to have one between her son and an inflatable pool toy. The son got fianceed sowmewhere along the line, but "you had to feel bad for the pool toy".
On ER in the first season finale, when Carol Hathaway's doctor fiance pulls out of the wedding at the last minute, her family goes through with the reception. As Carol put it, "What's a little humiliation when there's free food and drink?"
The Fresh Prince of Bel-Air: Will and Lisa decide not to get married, so Will's mother and Lisa's father get married instead. Which would have been veryawkward if Will and Lisa had been married...
On A Different World, Dwayne and Whitley got married at her wedding to Byron. When Jaleesa and Walter decide not to get married at their wedding, they have a big party instead.
Doctor Who, "The Runaway Bride": Donna mysteriously disappears from her own wedding. When the Doctor brings her back, she's furious to discover that the guests have decided to hold the reception without her.
Brian and Andrea do this on Dirty Sexy Money after yet another failed wedding for Karen.
When Bob Kate does a runner from her wedding to Lord Blackadder, Lord Melchett claims that it is traditional in these circumstances for the groom to marry the bridesmaid. The bridesmaid is Baldrick, who unlike Edmund seems quite happy with this.
In the third-season finale of Two and a Half Men, Charlie and Mia travel to Las Vegas to get married. Just before the ceremony, Mia tells Charlie that he'll have to kick Alan out of his house after they are married. Charlie refuses to do so and the wedding is off. Then, Alan and Kandi get married instead... which leads to Alan moving out of Charlie's house.
In Grey's Anatomy, Meredith and Derek seem finally ready to tie the knot, but opt out so the cancerous and dying Izzy can marry Alex before she becomes too weak to walk down the aisle.
In Scrubs, Turk misses his and Carla's wedding ceremony due to a mix up with directions. They end up going ahead with the reception, and stopping by the hospital later and getting a patient to officiate.
The same thing happened in Spin City almost ten years earlier. Paul and Claudia don't get to have a wedding ceremony because Mike mistakenly booked their wedding for the following week, and the Priest only agrees to marry them in the short time before a funeral is to take place in the same church, which is impossible for them. As a result they hold their reception and the Mayor, who has the legal authority to do so, marries them there.
in King of Queens, Arthur almost marries a 'famous' singer, Eva, but it turns out she only marries gay men, for companionship. Once she realized Arthur wasn't gay, she ran out on the wedding. Arthur, ever the penny-pincher, up and proposes to Spencer's mother. The marriage apparently lasts only a year.
Played with in Bones where Angela and Hodgins had to leave the altar because they found out that Angela was already married, thus leaving Brennan (the maid of Honour) and Booth (the best man) still standing at the altar. They look at each other and Booth even asks something along the lines of "What now?" Sadly, nothing happens, but it certainly amps up their Unresolved Sexual Tension and Ship Tease.
Turns into a Type C, with Hodgins advising the gathered guests to enjoy the catering while he and Angela run off together.
The 2010 finale of Home and Away uses this. Bianca runs out on her wedding, and Gina comes up with the idea.
Three's Company: After Larry's gold-digger bride runs off when she finds out he's not rich, the gang convinces Roper (who paid for the whole thing) to use it as an opportunity to renew his vows with his wife.
Subverted in the radio/television episode of Our Miss Brooks, "June Bride." When the French Teacher's "proxy wedding" gets cancelled, Mr. Boynton tells everyone that he has no intention of wasting the opportunity or the minister. Naturally, everybody, especially Miss Brooks, believes that he's finally going to propose. Instead, Mr. Boynton sees this as the perfectly opportunity for a square dance as he's Oblivious to Love.
In White Collar Peter went undercover and had to plan a wedding with a Black Widow. After the arrest is made, he uses the decorations and things to give his wife the wedding she'd always wanted.
In The Nanny Jocelyn was going to marry Nigel, but Fran realizes that she is really in love with her chauffeur, Lester. Eventually, the two of them realize it on Jocelyn's wedding day- and Jocelyn marries Lester instead.
In the musical Side Show, when Violet's big publicity event of a wedding fails to go off, her (conjoined twin) sister Daisy proposes to the man she's in love with on these grounds. He turns her down.
There is a ballet about a young woman called Lisette whose mother wants her to marry the much younger son of a rich widower. Lisette already has a boyfriend and at the rehearsal dinner he hides in her room but the two of them get caught by the guests together with Lisette in her underwear (she was changing for the dinner) and the mother is humiliated, calling the wedding off (to the delight of the two children). It then ends with her chatting up the father, remembering he was a widower...
In La Sylphide on the day of his wedding to Effie, James chases after the mysterious Sylph. A Wicked Witch tricks him into killing the Sylph and he wanders out of the forest to discover Effie leaving the Church married to another man.
May be implied in Dragon Quest VIII: the rulers of Argonia and Trodain had made a pact for their heirs to marry and unite the kingdoms. Unfortunately, while Princess Medea turned out well, Prince Charmles... wasn't a good match. This leads to Medea pulling a Runaway Bride during both endings; in the better one, however, Argonia has learned that the hero is the son of his long-lost brother and is utterly devoted to Medea, and reveals this to those gathered for the wedding ceremony. Instead of fleeing the chapel, the happy couple end up walking out together, arm-in-arm.
There was a variation in an episode of the Care Bears cartoon. Two of the villains were supposed to get married, but because they were villains, that didn't work out so well. Nonetheless, the Care Bears had agreed to host the wedding for them, and decided to just throw a party with the leftover food and decorations.
In the Futurama episode "Zapp Dingbat", Zapp Brannigan falls in love with Leela's recently divorced mother, Munda, and get married. The ceremony is interrupted by hostlile aliens, who were supposed to sign a peace treaty on that very day. The wedding is canceled when Munda discovers that Zapp was tricking the aliens, who then attack. Luckily, her husband Morris arrives to save the day, and in the end decide to remarry, taking advantage of all the preparations already in place.
In The Smurfs episode "Papa's Wedding Day", the intended wedding of Papa Smurf and Flowerbell the woodnymph was called off because she was revealed to be the infiltrator intending to have Papa Smurf fall in love with her so he could be captured by Balthazar, only to soon have a Heel-Face Turn and help the Smurfs rescue Papa Smurf from Balthazar. With a big wedding cake going to waste, Papa Smurf decides that they should all enjoy it together.