24 was notoriously written on the fly, with the writers starting each season with practically no concrete idea where the thing was going to end up. Notably averted by Season 7, due to the 2007 Writers Strike and a year-long delay, resulting in a much more cohesive, planned-out storyline for that season.
The 100 has twice filmed scenes (not merely planned, not merely written, actually filmed) where Jasper dies, and both times the writers decided against it at the last minute, either cutting his death scene before the episode went to air or using the next episode to explain how he actually survived.
The Season 4 premier of Arrow ended with a flash-forward of Oliver standing over someone's grave, openly weeping. The writers/creators admitted they had no idea which member of Team Arrow was in the grave when they started, and only finally settled on it being Laurel when they ran out of time and had to pick someone. This was yet another thing about the death that angered the fandom.
Halfway through Season 3 of Supergirl (2015), Andrew Kreisberg was fired after numerous sexual harassment accusations. Since he'd been the main architect behind the season's story and would still have to be paid if it was used, the other writers had to quickly whip up an alternate second half that received many accusations of being obviously made up as they went, most notably with how several episodes were spent building up to the arrival of the villain Pestilence, only for her to be anticlimactically disposed of just one episode after her arrival.
The crossover event Crisis on Infinite Earths (2019) was designed to be as big a DC TV event as the crew could possibly make it, with offers for appearances sent to just about every living actor who'd ever played a major role in a DC property. The downside of this is that they then had to wait to find out just who they would and wouldn't be getting before they could do any serious writing on the special... which also meant they couldn't offer any specifics of what the actors would actually be doing or even how much they'd be paid. Michael Rosenbaum, who played Lex Luthor in Smallville and voiced Wally West/the Flash in the DCAU, specifically stated that he declined the offer because it seemed so ridiculous to demand an answer from him ASAP while telling him nothing about what he'd actually be signing on for.
Barney & Friends: The first season fell into this category, according to writer Steve White in an episode of the Purple Tales Podcast. Steve and the first season's only other writer Mark Bernthal had only six days to write each episode.
Battlestar Galactica (2003): In its early seasons the show promised an intricate and mysterious backstory for the Cylons, as well as more mystery and intrigue for the Colonial fleet as they attempted to find the lost planet of Earth. The opening credits even teased "[The Cylons] have a plan ...". The writers would later admit what most fans suspected, that there was no such plan (the credits were mandated by the network) and that they had continuously written themselves into corners with less than graceful plotting (ex. the reveal of the Final Five Cylons, none of whom had been planned to be Cylons before Season 3, and them being different from the others, which was necessitated by the fact that only seven humanoid models ever appeared during the New Caprica occupation even though in the pilot miniseries Caprica-Six had stated there were twelve - but then the Final Five being unique Cylons without model numbers or duplicates necessitated the invention of a defunct Number Seven in the backstory, since Grace Park had already chosen Number Eight for her character/s during the previous season).
Further exemplified in the ending of the series, wherein they find a planet to live on that they name 'Earth' (because the series premise requires they eventually end up there), despite having found another planet that was unlivable named Earth previously (as part of the prior season's twist finale).
Then further when for no reason and with no buildup they elect to destroy all their technology by throwing it into the sun in order to allow the twist series ending to be that this all happened in the long long ago of our own world.
The creators of The Big Bang Theory have commented that every season they start with simply the next page. Chuck Lorre apparently has a motto that "This isn't Lost" and thus they keep everything at the moment and not holding out based on what they have planned. Although looking back it would be easy to believe they did plan out at least a season in advance, given the progression of Leonard and Penny's relationship in the first two seasons and how Howard started to mature in the second season which allows him to start a relationship with Bernadette in the third.
Breaking Bad. While the writing wasn't exactly free-styled episode-by-episode (barring Season 3), there were some notable instances:
Jesse Pinkman was supposed to die somehow in Season 1 until Aaron Paul's acting chops convinced everyone to keep him on, and he effectively became the Deuteragonist of the show. Hank was also supposed to die in the first season, but a writer's strike cut the season short and spared him for another few seasons. Walt Jr. was also supposed to die, but he ultimately survived until the end of the show unscathed.
Gus Fring was only supposed to be on for a few episodes in Season 3, but his actor (Giancarlo Esposito) demanded more. Hector was initially intended to be the main antagonist of the third season.
On that note, Gus Fring was created because Raymond Cruz (who plays Tuco) had to leave for his role on The Closer and was uncomfortable playing such a psychotic character for so long, asking that he be killed off. Tuco was supposed to be the villain all of Season 2.
When Walt is invited to Gus' house, we see toys on the ground and Gus says his kids are out of town for the night. Gus having kids is never mentioned again, nor is any partner ever mentioned, either in Breaking Bad or Better Call Saul, leading some fans to think he made them up to manipulate Walt. The writers have stated the scene was meant to open the possibility of exploring Gus' family in later episodes, but obviously it ended up as an Aborted Arc.
The character of Mike Ehrmantraut was invented because Bob Odenkirk (Saul's actor) was busy filming for How I Met Your Mother during production of "ABQ", so Mike was created to go clean up the drug paraphernalia and help Jesse cover for Jane's overdose, when it was originally meant to be Saul. The writers admitted in retrospective it was a better decision because they now think Saul doing that would've been out of character. Mike was also supposed to be only a one-shot character, but Jonathan Banks enjoyed the job so much he stuck around and became a major character.
Jesse getting evicted from his aunt's house by his parents was because the house the crew were filming in changed owners, who now disallowed the crew to film inside... until Season 3, at which point the plot of Saul and Jesse blackmailing Jesse's parents into selling the house back at a discounted price was created.
The Season 4 finale "Face-Off" was written as a possible, although still open-ended, series finale because at the time of production the producers did not know if they were going to be renewed for a fifth season.
When the M60 was introduced in Season 5, the writers had no idea how it would be used.
A more minor bit in the same development: Walt takes off the watch Jesse gave him and leaves it on top of a payphone, for literally no reason except that he hadn't been wearing it in the flash-forward scene.
This continues in the spinoff prequel series Better Call Saul. In one episode of Chris Hardwick's post-show series Talking Saul, Hardwick told a crew member that he looked forward to the explanation for why none of the Breaking Bad characters recognize Saul after his high profile in this series, causing a priceless Oh, Crap! reaction as the guy clearly realized on the spot what a corner they'd written themselves into.
Charmed: The most egregious example was probably the whole arc with Chris, a character who came from the future to Set Right What Once Went Wrong. The problem is, he kept changing the story of what went wrong and doing all sorts of morally questionable actions behind the Charmed Ones' backs. Eventually, the writers decided that he was Piper and Leo's Kid from the Future, here to stop his older brother Wyatt from turning evil. The reveal made a lot of these actions seem even weirder in hindsight as a result, though considering the Bad Future Chris came from and that he literally had no real idea what he was looking for, him doing reckless, questionable and borderline stupid things actually does make a degree of sense. Word of God seems to admit that they didn't know what his story was, having only decided on this course of action because Piper's actress, Holly Marie Combs, got pregnant; before that, Chris was going to turn out to be future Wyatt.
Doctor Who, partly by necessity. While an individual head-writer/producer might have individual plans, the show's Long Runner status means that this is a little bit necessary. For example, a lot of the things that have made the show so iconic, such as The Nth Doctor trope or the Daleks, weren't planned but added as they went along.
Especially notable is "The Ultimate Foe", the end of the "Trial of a Time Lord" story arc. Original writer Robert Holmes died before he could write the final story, and the team brought on to replace him were actually legally forbidden from knowing his intended ending—but they did have to use the locations scouted (including a round Victorian-era building) and the guest cast that had been hired for the aborted and banned ending. The two-part finale has a quite noticeably rushed script, with a Master subplot that ends up going nowhere and the bizarre reveal that the prosecutor the Valeyard was actually a future incarnation of the Doctor the whole time, which the show has basically just ignored ever since.
Many stories were rewritten at the last minute for various reasons. The Douglas Adams story "City of Death", for example, was originally written as a story set in the 1920s in Monte Carlo and Paris to be filmed mostly in studio, until production manager John Nathan-Turner crunched the numbers and announced that for almost the same money they could film on location in Paris with a skeleton crew, provided the setting was updated to the modern day (since there was no way they could afford to redress any of the French locations to look like they were in the twenties) and most of the scenes set in Monte Carlo were changed to Paris. The producer went for it, and the whole story had to be urgently rewritten top to bottom mere days before shooting to reflect a modern-day Parisian setting, using the cast already hired for the original script. (This is one of the reasons the story-only companion, Duggan, seems more or less extraneous in the final version.)
Russell T Davies, the first showrunner of the revival series, has openly stated that he tends to write overarching stories (particularly ones that span multiple seasons) as he goes, rather than having it meticulously planned out in advance. This often resulted in him running with things that people pointed out to him:
The fact that the Doctor happened to have an encounter with Donna, then coincidentally ran into her grandfather the following Christmas, then happened to run into Donna again after they, by total happenstance, decided to investigate the same suspicious company. Davies has said that someone brought up how coincidental it was, and wondered if there was some reason for it. At the time, which was before he had finished finalising "Partners in Crime" he had nothing planned. He realised that it looked like he did though, and began tying it into the series' overarching narrative. The point about Donna's grandfather being encountered was also, according to the DVD Commentary, something that Davies only realized after he was asked if they should recast Cribbins in the Christmas episode if he was to be Donna's grandfather but Davies thought it'd be more interesting if they were the same character. For this reason, they also took the hints that Donna's fate seems cosmically interwoven with the Doctor's thing up a notch in that episode.
Similarly, when writing "Planet of the Ood", Davies put in the Ood calling the Doctor and Donna "the Doctor-Donna" just because he thought it sounded cool.Cue the finale, in which Donna becomes half-Donna half-Doctor due to a Time Lord-human metacrisis. Davies didn't realize that he had unintentionally created some major foreshadowing until someone pointed it out to him, thinking he had done it intentionally. He then rewrote "Journey's End" to include a moment of the Doctor remembering that the Ood had called them Doctor-Donna.
One notable subversion for Davies was the overarching story concerning the Doctor's severed hand, which went from the 2005 Christmas special to the Series 4 finale in 2008. Davies claims that since the moment he wrote it into the script for "The Christmas Invasion", he knew that he would end up making a "second Doctor" grow out of the severed hand, whom the original Doctor would send off to live happily with Rose. He even told David Tennant about these plans at the time.
Davies openly declared after Series 3 that the mysterious shot of a woman's hand picking up the Master's ring had no intended meaning; he had no further plans for the character, but with his time as showrunner coming to an end he figured he should put in some kind of Sequel Hook to make it easier for future writers who wanted to use him again. Then in his very last episode, he decided to follow up on it himself after all.
Davies admitted in a Production Notes column for Doctor Who Magazine that he came up with the "Bad Wolf" Arc Words on a whim while writing "Aliens of London" after he wrote down that the TARDIS got vandalized with graffiti as a joke, he then later idly thought it'd be neat if the words would keep popping up throughout space and time as a phrase and started rewriting other episodes to start featuring them. Then the production team took notice and started inserting the phrase into more episodes themselves despite having zero clue what "Bad Wolf" actually meant. It all paid off in the finale, which Davies decided would be responsible for the words popping up throughout the season.
Julian Fellowes only wrote the first half of each series of Downton Abbey ahead of time, then waits to see how the actors play off each other before writing the second half. Occasionally, this bit him in the ass when he decided to expand a character's role only to discover the actor wasn't available, most notably with Edith's paramour Gregson.
Many committee-led series will change plot and emphasise characters depending on audience responses to broadcast episodes. Sylar and Hiro in Heroes received such a favourable response they were given much larger roles in the long run including Sylar being allowed to live beyond the Season 1 finale.
How I Met Your Mother starts with a Driving Question of "How does Ted meet the future mother of his children?" and the creators were adamant that it would happen in the Grand Finale. The thing is they weren't sure how long they would get to tell this story and there were several admitted plans in place in case they were or were not renewed. Victoria of season one was outright confirmed to be the Mother in case the initial 13-episode order was all they had. Stella is theorized to also be a back-up Mother (given how they meet ties directly into the implied "Myth Arc") if season three was the end. Once ratings stabilized it seemed that the writers had a clear idea of how the show would end by introducing some more solid clues (the 100th episode has Ted meeting the Mother's roommate and getting a lot of, still vague, information on her) but still had to keep things flexible because now they weren't sure when they were ending.
Parodied in a The Kids in the Hall sketch, which warns the viewer that it was "written in haste," showing the writer frantically mashing a keyboard trying to finish it within the deadline. The scene is filled with nonsensical actions and garbled dialogue caused by the typos, such as a man taking off his "rubber boobs" and sitting down on a "chain."
The late 1940s TV show Kukla, Fran and Ollie thrived on this. The show was completely unscripted and the actors ad-libbed everything on set.
In the original KTMA season of Mystery Science Theater 3000, the riffs were improvised rather than written. The films being mocked hadn't even been seen in their entirety in advance. In the Comedy Central era, however, each film was carefully screened and written before its respective episode was recorded. The show became better for it.
During the first season, movies weren't watched in their entirety until the actual writing process took place. They changed to pre-screening the whole film after viewing the violent rape scene in The Sidehackers.
NYPD Blue was often written on set during filming due to head writer David Milch's drug use. "OK, you [Sipowitz] say this, and then you [Simone] say this in response." Actors wrote down their lines on scraps of paper in the squad room. Jimmy Smits and other actors quit over the hectic scheduling.
The series finale of The Prisoner (1967) was written in a trailer over a weekend. Not surprising that it's one of the most infamous Mind Screws in television history.
One Foot in the Grave: David Renwick was asked how he made decisions like the fact the Meldrews hadn't had any children (except a son that had died long before the series begins). He replied that he only was writing an episode at a time and thinking of what worked in the moment; he never decided they didn't have grown-up kids out there, he just never thought of an episode where it was required that they did, but did write an episode where Stuart's existence made sense, after which they couldn't have.
The Season VI finale "Out of Time" was extremely rushed, and in fact, on the night it was filmed before the live audience it still wasn't entirely complete, meaning the writers had to type the script directly onto autocues for the cast.
Writer Doug Naylor repeatedly dithered over what the ending to Series VIII should be, having had to scrap his originally planned finale as the budget had run out. The ending they went with was so rushed the director had to step in to play a part using a costume nicked from another series; this replaced another ending in which the cast was purportedly in costume, ready to film when it was scrapped.
Series X had all six scripts written and ready to go... but then the production found out they wouldn't be able to do any location filming (it being a choice between that and having a live audience for the studio records). Episodes 5 & 6 couldn't go ahead without the location filming, and they were both scrapped and had to be replaced with new episodes, written whilst the other four were being filmed. Only half of the new episode 5, "Dear Dave", was able to be filmed in front of an audience because that was all that had been written, and they had to go back later, shoot new scenes with greenscreen and splice it all together. At the time the cast was being interviewed for the making-of documentary for the DVD, they still weren't sure if they were going to be able to film everything.
To compound this, the ending of episode 4 required a chimp, who would be played by an actor in a costume. Nobody realised that there were limits on how many hours he could work inside the chimp suit until the day of filming, meaning the original ending had to be thrown out and a new one written more or less on the spot.
Saturday Night Live is well-known for this as well, particularly in regards to its earlier years, being broadcast live and all. Most famously, sometimes writers would hide under the "Weekend Update" desk and hand new jokes to Chevy Chase on the air.
Star Trek: The Original Series: Newly-hired showrunner Gene L. Coon cemented his legendary place in Trek history by writing, as his very first script for the series, the classic episode "Arena", over just one weekend when it was determined that there no other scripts ready to put into production. Although the episode is credited as adaptation of the Fredric Brown short story of the same name, the similarities were not noticed until the show's researcher Kellam de Forest pointed it out, and Desilu paid Brown to "adapt" his story and gave him a story credit. Meanwhile, not to be outdone, newly-hired director Joseph Pevney cemented his legend when he turned the episode around in just six shooting days, one day ahead of the planned schedule.
Star Trek: The Next Generation: The famous cliffhanger at the end of "The Best of Both Worlds Part I" was due to the writers not knowing if Patrick Stewart would return as Picard or not, forcing them to pause there and wait to write Part II by the seat of the pants once they found out about Stewart. Amusingly, Michael Piller, who wrote the finished script, had also decided to leave the show, and thought it was quite funny that he would be leaving the resolution to somebody else... and then he decided to stay on too, and suddenly had to come up with a resolution himself! Luckily, this turned out extremely well and the two-parter is still considered one of the entire Trek franchise's all-time greatest moments. Not so luckily, this encouraged the writers to keep on doing it at the end of each season on every Trek show from then on, which naturally had increasingly diminishing returns.
Supernatural: The writers were pretty open about their habit of throwing stuff at the wall and seeing what stuck. If fans reacted badly to plots or characters, they would often, for better or worse, change course.
Creator Eric Kripke had a plan for the ending of Season 3 which was derailed by a writers' strike which cut the season short. This led to the season ending with a massive cliffhanger, Dean dying and going to Hell. Kripke had an arc planned that would have had Sam save his brother, but there was no time to set it up. So, Season 4 introduced the concept of angels, and specifically, Castiel, the angel who pulled Dean from Hell. Prior to that, Kripke had been adamant that angels would not appear on the show even though there were demons. Season 4 gave us the angels and made them Old Testament-style "dicks" who didn't care much for humans.
After revamping the mythology to include angels, the writers created a female love interest for Dean who was a Fallen Angel, but audiences didn't warm to the character. So, they scrapped that character and gave most of her arc to Castiel, who did not die as planned. This led to creating an entire backstory for Castiel's vessel as well as making him Dean's Not Love Interest. Or maybe his Not Not Love Interest. They also retconned stuff from earlier seasons, such as making The Trickster really the archangel Gabriel.
Season 5 had been planned to be the end of the show, so Season 6, under a new showrunner, had to undo the finality of the previous season. Sam returned from Hell soulless but that whole arc was scrapped mid-season and as a replacement, Castiel became the Big Bad.
The whole Leviathan arc was a last-minute replacement in Season 7 because the war between Heaven and Hell that was planned was either too expensive or wasn't working on the page, depending on whom you talk to. The Leviathans ended up not being very popular with fans because for all that they were hyped up as primordial entities so bad that God locked them away, their main things were shapeshifting and eating people, both of which were already covered by other monsters on the show.
Season 10 was thought to be the end for the show, then it was renewed, so halfway through that season the entire Mark of Cain story changes.
The series has many examples of Throw It In! and such but the identity of the killer wasn't meant to be revealed in the first place and happened only because of the network forcing Lynch and Frost's hands. This, of course, doesn't make it any less this trope, or rather it could be considered an even better example.
After revealing who killed Laura Palmer they didn't have a clear idea how to keep Agent Cooper in the series, and with Lynch busy on over projects, the writers struggled throughout much of series two for the show to have a focus.
Aaron Sorkin does this. There's a story that when he was writing The West Wing, he needed President Bartlet to be lying in bed for a scene — and so gave the character multiple sclerosis.
The X-Files: While the series' Myth Arc is known for suffering from The Chris Carter Effect, making it this, the trope is in effect in a number of Monster of the Week episodes as well, as many times, the solution to the current mystery contradicts the established evidence or fails to explain some of the events that unfolded, leaving them as Big Lipped Alligator Moments.
Specials:
David Bowie's unlikely guest appearance on Bing Crosby's Merrie Olde Christmas TV special in 1977 was cobbled together in a hurry. The show was taped in London and the producers were scrambling to find a guest star who could draw in younger viewers. Someone informed them that Bowie lived close by, and when he was contacted he agreed to come in on short notice (they enticed him in part by agreeing to play the video for his then-current single "Heroes" on the show). The script called for Bowie and Crosby to duet on "The Little Drummer Boy". Bowie said he hated that song and refused to do it. Caught in a jam, the show's writers took a 90-minute break and wrote a new song called "Peace on Earth" for Bowie to sing in counterpoint while Crosby sang "Drummer Boy". After less than an hour of rehearsal, Bowie and Crosby taped the final piece that became a classic.
