The Adventures of Pinocchio was supposed to end with Pinocchio getting killed off for being such a sociopa... er, "bad little boy". Carlo Collodi's editor forced it so that Pinocchio was saved from death, and 20 more chapters were written.
One editor demanded of Daniel Keyes that his "Flowers for Algernon" have a happy ending. Every writer Keyes asked about it told him to refuse.
Spellfire became infamous for editor-carnage, leaving many plot threads ripped apart and dangling and characters looking like idiots or jerkasses.
The Double Diamond Triangle Saga. The idea of nine books that can be read in different orders was interesting, but ran into the ground at supersonic speed. How long do you think authors were given to sync all possible criss-crossing plots and write it? Read a little revelation from Ed Greenwod's spokeslady. And if he is given this much care...
In an odd case, TSR told R.A. Salvatore to bring back Wulfgar or they would do have someone else do it for them. What makes it odd is that Drizzt was clearly the breakout hit character, with Wulfgar as a mostly unneeded sidekick. Salvatore did the best he could, though, and in the end got some good stories out of it and wrote the character out again (this time much as he had written out the barbarian tribes the character belonged to shortly after the character's "death").
In another case, Salvatore was told that he would have to kill off Artemis Entreri, as the game was eliminating the assassin class and all assassins in the setting were going to be killed off as part of a ritual to empower the dark god Bane. Salvatore, not wanting to lose a good character, countered that Entreri wasn't an "assassin", but a fighter/thief who killed people for money. TSR backed off. (Later, Wizards brought back assassins as a prestige class for 3rd Edition. Entreri had one level in it.)
Thornhold was supposed to be the first book of its own series, not glued to Songs and Swords on the side. So when it was shut off, many a Sequel Hook was left dangling in midair and readers screamed "Arr!" Elaine Cunningham later used short stories for "Best of the Realms" to close at least a few of them.
Neil Gaiman's "The Goldfish Pool and Other Stories", is about Executive meddling, plus truly epic amounts of Adaptation Decay. A writer is called in to Hollywood to work on a film adaptation of his hit novel, Sons of Man, which is a speculative story about Charles Manson being possessed by a demon, and the children he sired of the women in the Manson Family coming under the power of that demon, with a sole daughter he had trying to stop them. By the time the ever-changing Hollywood executives are done with it, it's a slasher plot called When we were Badd about a serial killer named Jack Badd who possessed a video game after execution and possessed the kids who played it, with the now-male protagonist saving the day by burning the electric chair the killer was executed in.
It ends with the writer's offhand quote of a song lyric being taken out of context and used as the plot for a completely different movie about a woman trapped in a loveless marriage. At that point he gives up and goes home.
Gaiman has a long history of Hollywood stresses: in addition to the aforementioned Jon Peters run at Sandman (resulting in what Neil described as "the worst script I've ever read"), he was approached on an adaptation of Anansi Boys. Y'know, the book where the main characters are the children of African deity Anansi, and thus they (and a lot of their acquaintances) are black. The first question asked was apparently, "Is there any way we can make them white?"
"So," Zahn said, with absolutely the driest expression and tone of voice you ever saw, "I pointed out that way back in Heir to the Empire, Artoo flew the X-wing to Coruscant on his own. 'But Artoo can't fly the X-wing.' Okay, in ESB, one of your own movies, Luke's X-wing was inside the Hoth base, but Luke meets it outside. 'But Artoo can't fly the X-wing.' Then later in the movie, Luke tells Artoo, "No thanks, I'll keep it on manual for a while." Manual indicates that there must be an automatic. 'But Artoo can't fly the X-wing.'"
"Finally," he said, "I figured out what it was that bothered them. It wasn't Artoo flying the X-wing; it was his docking the ship with the Starry Ice without the aid of tractors." He added exactly three words to the existing scene, with Faughn now telling Luke that the Starry Ice had a pair of half-ports "with tractor assists". The editors were happy, and all was well.
One that did work out was that Lucasfilm shot down Timothy Zahn's original name for the Noghri which was Sith, to explain why Darth Vader was known as the Lord of the Sith. The reason, of course, was that Zahn didn't realize that George Lucas already had something very different in mind for the meaning of that title.
Later events made fans theorize that George Lucas realized that he had virtually identical stories for Anakin Skywalker and Anakin Solo (hope of the Jedi Order, deep connection with the Force, skill with technology, and oh yeah, a fall to the Dark Side that plunges the Galaxy into war, destroying a republic). So in the end, they killed off Anakin Solo and gave his fall to the Dark Side plot to older brother Jacen.
Legendarily, Robert A. Heinlein's Stranger in a Strange Land was pared for both size and content to meet publisher demands. Thankfully, the editing was done meticulously by Heinlein himself, so the novel came out more or less as intended. Following his death, the unedited version was released by Heinlein's widow.
The British publisher did their own meddling on the first book. They were the ones who insisted the author go by J. K. Rowling since it was felt boys wouldn't want to read a book written by a woman. They also wanted to cut the troll scene where Harry and Ron save Hermione.
Lindsey Davis's Falco novels suffered similar fates of Americanization, or would have, had she not been adamant. See here for her very funny article about it all. Read "A Gentle Corny Rant" on this page.
Terry Goodkind's book Wizard's First Rule had at least one instance of the use of the titular rule scratched out by the editor. Maybe it was for the better, since we all know where it went from there...
Umberto Eco once wrote a humorous piece, "Editorial Revision", about (fictitious) editorial changes for the better in famous literary works; for example, "The Waste Land" by T. S. Eliot originally started like this: "April is the cruellest month. And March isn't all that great, either."
Ironically enough, "The Waste Land" itself was heavily cut down at the suggestion of Ezra Pound. It's been said that while Eliot wrote the work it was Pound who found it, and the former gratefully acknowledged him as "il miglior fabbro"note "the better craftsman", in Italian in the poem's dedication.
In a minor example, Jim Butcher originally wanted to call the first book of The Dresden FilesSemiautomagic. For whatever reason, the editor or publisher didn't like it, and so he called it Storm Front instead, creating the trend of two word titles with each word having the same number of letters.
His original proposed title for the thirteenth novel was Dead, but this was rejected on account of being a Spoiler Title. He eventually went with the more ambiguous Ghost Story instead.
Butcher had initially planned for the necromancers of Dead Beat to show up in the 8th novel, after the events of Proven Guilty. As the 7th book was the first to be released in hardback, his publisher strongly urged him to include something "truly spectacular" in it, so he switched those two novels' events and brought the zombie tyrannosaur out early. A rare case where Executive Meddlingdelighted the fans.
A positive example: Isaac Asimov wrote a story in which energy beings come to Earth and purchase Jupiter from humanity to use as an advertising billboard for their passing ships. Asimov titled the story "It Pays", but an editor, without consulting Asimov, changed it to "Buy Jupiter". Asimov, being the punster that he was, liked it so much he used it as the title for the paperback collection which contained the story.
Another Asimov example features his most famous work, I Robot. The first story, "Robbie", was originally titled "Strange Playfellow", but was changed and the collection itself had a different name until Asimov was convinced to make it I, Robot (there was already another work by another author with the same name, and he felt bad about taking the name, but his publisher convinced him otherwise).
Jules Verne initially planned for Captain Nemo of Twenty Thousand Leagues Under the Sea to be a Polish aristocrat who was fighting Russians after his family was killed by Russian soldiers during the ill-fated January Uprising. His editor, Pierre-Jules Hetzel feared that this might cause diplomatic problems because Russia was allied with France at the time. With Franco-Prussian War looming on the horizon, Verne was persuaded to make Nemo a mysterious stranger fighting the British and later, in The Mysterious Island, he revealed him to be an Indian prince while retaining motivation (family lost during the brutal quelling of Indian Uprising).
The German publisher "Aufbau Verlag" loves meddling with their translations of classics. Most glaring example is Victor Hugo's Les Misérables - from somewhat about 1400 pages (in original length of the German translation) they left about 900. They cut out Valjean's theft on Petit Gervais, several of his My God, What Have I Done? moments, a good chunk of Marius's backstory, the characterization of several important minor characters... and also quite a few of Hugo's Author Tracts.
The Twilight Saga was originally supposed to be two books long, going straight from Twilight to Breaking Dawn, which at that time had the working title of Forever Dawn. It was Stephenie Meyer's editor who suggested that Bella's senior year be drawn out, resulting in New Moon and Eclipse. (It was also suggested that Forever Dawn was "inappropriate" for Young Adult audiences.)
The Jungle by Upton Sinclair was subjected to this by Doubleday. The original edition published by the socialist newspaper Appeal to Reason was singularly focused on the hell the main characters, an immigrant family, go through because they are poor, are not fluent in English, and because the businessmen run everything and was obviously meant to make readers see the horrors of wage slavery. Doubleday, however, was a corporation, and it therefore didn't take too kindly to The Jungle's criticism of corporations and the ways in which they exploited their workers. They forced him to make the family less ethnic, amend some passages so that their lives were not so unfair, and make the passages about the tainted food more graphic. Most people read Doubleday's edition and they were singularly focused on the unsanitary conditions of the factories making food products and the book is now taught as an exposure of the corruption that led to tainted food. The only hint that American history textbooks will give as to the disparity between Sinclair's intentions and the public's response is his quote "I aimed at the public's heart and by accident hit its stomach."
In probably one of the most heinous examples, L.J. Smith was fired from writing her own series. The article speculates that this was because they didn't like the romantic pairing she was planning on going with in the ending, but no reason has been officially given. Shipping: Serious Business.
This is why there are two versions of Stephen King's The Stand. The publisher thought it was too long and asked him to cut it down. Granted, a few scenes, like 'the Zoo' and the gay rape scene between The Kid and Trashcan Man would never have flown in the seventies, but other stuff was just seen as making the story too long. He put back much of the cut material (save a few parts he thought belonged cut) back in for the Complete Uncut edition.
James Gurney's third Dinotopia book, 'First Flight', is different from the others because the publishers wanted it targeted toward younger children than the other books. It didn't go over well with the fans and had some plot problems.
Weirdly there was a reverse when Anthony Burgess's A Clockwork Orange was released in America. The publisher's removed the final chapter when Alex redeems himself and decides to "grow up" and changed it with a much darker ending. They believed it would be a more realistic ending and that US audiences would prefer that. So in this case it was a happy ending being changed into an unhappy ending. This has led to endless debates over which version of the book is better (the one with chapter 21 or the one without it).
Enders Game provides an in-universe example: in order to train Ender into an excellent commander, his teachers slant the odds against him in every single way possible. They start by making him commander of Dragon Army, which had been discontinued due to how awfully it did in the games, and give him an army of total newbies who have never fought before in their lives. To add on to this, they give him a mere three weeks to train his army instead of the usual three months, and once training is over, they make him fight a battle every day instead of one every two weeks, and when that doesn't break him, they give him two battles every day. Ender, being who he is, manages to overcome every obstacle and even top the leader boards with standings never thought possible. This hits its peak when they put a giant barricade in front of his entrance into the battle area (preventing him from seeing the rest of the arena) and pitting him against two armies at once. After he manages to beat that, they transfer every single toon leader (leaders of mini armies inside the main army) to another army. This doesn't matter, as Ender was transferred to command school directly afterwards.
In a rare case of executive meddling gone right, editors persuaded Derek Landy not to kill off Tanith Low in the first Skulduggery Pleasant novel due to her further potential as a character to develop and to act as a mentor to Stephanie in ways Skulduggery perhaps couldn't be. Landy got his revenge, agreeing to keep her alive for book 2 on the condition that for every book she lives he gets to torture her.
The Lord of the Rings was actually the result of Executive Meddling. After the success of The Hobbit, Tolkien was asked to write a sequel. His first choice was the tales of the Elder Days he had been working on in various states of development since World War I, and would eventually become The Silmarillion. His publisher, however, insisted that the new story have more about hobbits. The end result was reediting The Hobbit to fit it into Middle-earth and connect it to the larger Legendarium, and the writing of the defining work of modernEpic Fantasy.
Editors also broke up Tolkien's original single massive work into three volumes for economic reasons. Three-volume novels date back to Victorian times, but these were more like serials than today's fantasy trilogies.
Editors were frequently trying to correct the language used by Dashiell Hammett and Black Mask were actually censoring perfectly innocuous phrases like "Gooseberry Lay," which meant something like stealing washing from washing lines.
Hammett put that phrase in on purpose, guessing - correctly - that the censors would zero in on it and overlook "gunsel", which at the time was not a synonym for "gunman".
Alfred Knopf kept trying to change Raymond Chandler's titles, "Farewell My Lovely" in particular.
James Ellroy's publishers asked to cut his 900 page White Jazz in half. Their intention was, presumably, that he cut a couple of subplots, and not, as he did, take out pretty much all words that weren't absolutely vital to understanding the sentence, resulting in a quick, telegraph-like staccato style, for example, "Time revoked/fever dreams-I wake up reaching, afraid I'll forget. Pictures keep the woman young. L.A., fall 1958. Newsprint: link the dots. Names, events-so brutal they beg to be connected. Years down-the story stays dispersed.".
The reason they wanted Ellroy to cut down his page length was because they felt he, with eight books and high praise for "The Black Dahlia," wasn't a big enough name to justify a 900 page book.
Executive meddling helped authors Larry Niven and Jerry Pournelle for the better. The two had presented their publisher with an alien invasion novel in which the invaders strike Earth with an asteroid in order to soften up humanity. The publisher insisted that the asteroid impact would make a great story all on its own and that while alien invasions were a dime-a-dozen, he'd pay big bucks for the asteroid story. So they went back home, removed the alien invasion, and came back later with the novel Lucifer's Hammer. They eventually published the alien invasion story as Footfall.
John W Campbell Jr, editor of Astounding/Analog SF magazine, was famous for working with his authors, often demanding full rewrites that even the authors had to admit improved the stories.
Campbell pointed out to Isaac Asimov that some of his stories implied the Three Laws of Robotics, which Asimov hadn't actually realised himself.
He also pointed out to Robert A. Heinlein that his stories formed a "Future History," although Heinlein didn't care for the term as a label for his work. It probably helped as well that Campbell was an established sci-fi author himself, having wrote Who Goes There? (the basis for The Thing (1982)) and therefore knew what he was doing, making this something like an Averted trope.
Campbell's most infamous author-editor exchange was The Cold Equations. He wouldn't publish it unless it ended with the girl's death.
Donald Hamilton wrote a thriller in which the protagonist was killed at the end. His editor suggested revising it so that the character lived, and making a series out of it. The book was Death of a Citizen, and the protagonist was Matt Helm, which became a successful long-running series.
Papillon first draft was a compilation of tales about inmates trying to escape the French Guiana penal colony during the Thirties and the Forties. The publisher convinced Henri Charrière (former prisoner of the French penal colony himself) to rewrite the text as an autobiography, making him becoming an Unreliable Narrator.
Donaldson's original title for the first book was Foul's Ritual. At the prompting of his editor Lester Del Rey, it was changed to the more striking Lord Foul's Bane.
Initially, the second book (The Illearth War) contained a lengthy section from the point of view of one of the the Bloodguard. Del Rey told Donaldson to remove the section, as such a substantial subplot from the viewpoint of a native of the Land wreaked havoc with the ambiguity of whether the Land was real or not. When he realized what he'd done, Donaldson apparently agreed wholeheartedly (though the cut section remains available on his website).
The Second Chronicles of Thomas Covenant was originally supposed to be four books and the title of the first was Sunbane. Del Rey convinced Donaldson to cut it down to a trilogy (like the First Chronicles) and came up with a new title for book one (The Wounded Land).
A case of executive meddling that failed involved Del Rey becoming upset that the Second Chronicles were revolving less and less around Covenant's own POV and more around new protagonist Linden Avery's. When confronted about it, Del Rey's reasoning was that you couldn't do good worldbuilding with a female narrator... for some reason. Considering that Linden's arc was at least as important (if not moreso) than Covenant's in the Second Chronicles, Donaldson fought for this one, and ended up getting moved to another editor who let him keep Linden as the primary POV character.
When A.S. Byatt's novel Possession was brought to the U.S., her publisher insisted that she cut sections of description and poetry and asked her to add sex scenes. Byatt refused, giving in to only one demand: a longer and more effusive description of the hero, intended to make him more attractive to American readers.
That story also has an In-Universe example of this: Mort Cropper's biography of R.H. Ash reads more like Cropper's own autobiography. And averted: James Blackadder, as a young man, was trained by a spiteful English professor to believe that his own ideas and opinions were worthless. As the editor of Ash's Complete Works, Blackadder allows only background information that can be reliably sourced.
Also from the literary-fiction field, the first version of Raymond Carver's short story "A Small, Good Thing" was heavily edited by Gordon Lish (infamous for this sort of thing) to a shorter version with much sparer prose and an Ambiguous Ending, then published as "The Bath." Later Carver's original was published. There's an ongoing debate as to which version is better.