Doctor Who, over its decades of existence, has had plenty of Throw It In! moments.
- William Hartnell's habit of flubbing his lines was left in due to the limited budget (specifically, the difficulty and expense of editing the 2" tape used to record the productions, limiting the tapes' mandated reuse). If you watch carefully, many early episodes will be peppered with minor flubs, usually a slight but noticeable delay or stutter. On a few occasions, you might even hear Hartnell correct himself on camera, restating a word he'd misspoken.
- William Russell was able to ad-lib so well off of Hartnell's frequent hashings of his character's name that it was eventually written in the scripts that the Doctor would mispronounce it.
- Viewers in the 1960s became so used to errors being left in (something that was not unique to Doctor Who back in the day) that when Ian begins choking in an episode of "The Reign of Terror", concerned viewers thought they'd actually recorded the actor having a choking fit (but it was scripted).
- This is referenced in the video game The Eternity Clock, in which River writes in her diary about going back in time to meet the First Doctor, and she transcribes his words to her as containing several "flubs", restarting his sentence and mis-ordering his words. She sarcastically comments that the Doctor had had more erudite selves.
- This is also referenced in the Doctor Who Missing Adventures First Doctor book The Plotters, where the Doctor often misspeaks — but the garblings are used for Leaning on the Fourth Wall. (For instance, when he has his suspicions of alien activity quashed in what had previously been a pure historical story, he admits "For one moment there, I thought this episode — I mean to say this episode of my life — was going in a different direction.") The book caused some backlash upon being released because of this, as some fans found the inclusion of "Billy-fluffs" to be disrespectful.
- Patrick Troughton frequently varied his lines or added unscripted business during his time as the Doctor, including the first ever (albeit non-romantic) Doctor/companion kiss.
- In "The Seeds of Death", Zoe pulls open a door and the Doctor comes bursting in on a wave of "toxic fungus" (represented by soap foam). He makes as if to dash off and then promptly slips on the soapy floor and falls over onto his face. This was not supposed to happen and, while Patrick Troughton doesn't break character in the least, Wendy Padbury, who plays Zoe, can be seen Corpsing her face off for the rest of the shot.
- The Shower Scene in "Spearhead from Space" happened because the house The BBC had rented to shoot in had a truly amazing old-fashioned shower in it that everyone decided was too good not to use. A scene was altered so it could happen while the Doctor was in the shower, providing the show's first proper Doctor Costume-Test Montage (a tradition ever since), while incidentally revealing that the Third Doctor (via Jon Pertwee's navy service) had an unexplained tattoo.
- In the scene with the Auton fake policemen in "Terror of the Autons", a stunt performer was accidentally hit by a car (it was supposed to stop a few inches away from him, with the impact implied by an off-screen sound effect) and knocked all the way to the bottom of a quarry. Since the stunt artist wasn't seriously injured, the spectacular fall was left in the finished show.
- Almost every time the Fourth Doctor steps on his scarf or gets it stuck in something, it was a genuine mistake caused by how insanely impractical the outfit was to wear, but it was all left in because Tom Baker was extremely good at not breaking character and it fits the Fourth Doctor's Cloudcuckoolander personality to be constantly locked in a struggle with his own clothes.
- The scarf itself is an example. The producers wanted a scarf to make the new Doctor look unlike the Third, and arranged for a woman to knit one. Not knowing how much wool they'd need, they bought what they were sure was more than enough, expecting the woman to only use what she needed. Instead, she used the lot. But the moment Baker tried it on, they knew it was perfect.
- In "Pyramids of Mars", there's a funny bit where Sarah and the Doctor enter a room where a mummy has its back to them and immediately turn around and head back out without apparently reacting to it, in a The Marx Brothers-like manner. This was a suggestion from Tom Baker and Elisabeth Sladen — the director had told them it was a funny idea but they didn't have enough time to rehearse it, so Tom and Lis did it on the first take anyway, nailed it, and it went in.
- Baker and Sladen notoriously rewrote and ad-libbed most of their material in "The Android Invasion" due to hating the script, including the entire ending scene. Some are obvious unrehearsed adlibs, which have a noticeably more natural, mumbly feel than Who acting usually does (like the Doctor and Sarah's "fee fi fo fum" bit). The original dialogue was at best functional — the rewritten dialogue is much odder and more exciting as well as showing off how well-read Tom Baker actually was when making off-the-cuff references to Anna Karenina and namedropping obscure historical figures.
- In "The Hand of Fear": after Sarah has been freed from Eldrad's mind control, she says "Eldrad must live!... Just testing." That was an ad-lib.
- In "The Deadly Assassin", there's a part where the Doctor is sitting in a chair being lectured by a Time Lord, and suddenly stands up with a stormy expression on his face, looming over the other man and causing him to falter at the end of his sentence as if afraid. This was an ad-lib from Tom Baker.
- In "The Robots of Death", Tom Baker changed the in-universe name for the Uncanny Valley effect ("Gimwol's Syndrome") to "Grimwade's Syndrome", turning it into an affectionate jab at a production assistant notorious for always ending up working on robot-related stories.
- Leela kissing Adam Colby on the cheek in "Image of the Fendahl" was ad-libbed by Louise Jameson. The surprised look on his face is genuine.
- Soldeed in "The Horns of Nimon" begins laughing hysterically during his death scene, while yelling "you are all doomed! All doomed!" The line was scripted. The laughter was (nearly literal) Corpsing. The actor had thought it was a camera rehearsal, but the take got used.
- "The Caves of Androzani" features several scenes of the villain Morgus turning to the camera and making Aside Comments. The first time this happened was because the actor misunderstood the stage directions, but the director decided to keep it in because it felt Shakespearean and tied into the plot's resemblance to early-modern revenge tragedies.
- Colin Baker was personally responsible for several notorious puns during his time as the Doctor, including the "Perrier water" joke (punning on the name of his companion Peri) in "Vengeance on Varos", and the Doctor's "No 'arm in trying" in "Revelation of the Daleks", after Davros' hand is blown off.
- Russell T Davies, the first showrunner for the revised 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 have pointed out to him:
- The fact that the Doctor happened to have an encounter with Donna, then coincidentally ran into her grandfather during 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. For this reason, they also took the hints that Donna's fate seems cosmically interwoven with the Doctor's thing up to eleven 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 the finale, "Journey's End", to include a moment of the Doctor remembering that the Ood had called them Doctor-Donna.
- In "Tooth and Claw", the Doctor and Rose were both supposed to fake Scottish accents, and then forget to use them when the action started. David Tennant, who is Scottish, could do a fake Scots accent just fine, but Billie Piper could not, so a scene was added where Rose tries and fails to do a Scottish accent that is so terrible the Doctor tells her to knock it off.
- In "Planet of the Dead", the bus prop got heavily damaged during shipping to Dubai, so Russell T. Davies added an exchange in which the Doctor explains the bus got damaged by passing through the Negative Space Wedgie. (It doesn't make a lot of sense if you think about it too hard since the premise is that the bus's immunity to wormhole damage shields the living people inside... but considering the circumstances, it's reasonable.)
- The Doctor ripping off a strap in "The Time of Angels". The first take was an accident, but the producers loved it so they told Matt Smith to rip it off in subsequent takes.
- The Doctor/Rory snog in the episode "Dinosaurs on a Spaceship" was completely ad-libbed by Matt Smith. So yes, Arthur Darvill was just as surprised as he looked.
- Matt Smith had shaved his head for a role in the film Lost River, and it did not grow back in time to reform his Doctor's signature coif for his final episode "The Time of the Doctor". He shaved his head again for a bald cap and donned a wig. A script re-write incorporated the wig into the story, leading to several of the episode's funniest moments.
- In case you think on-camera errors disappeared after the 1960s, not once but twice during the Capaldi era the lead actors have been captured sneezing on camera, yet it's been left in. The first is in the 2014 special "Last Christmas", when Peter Capaldi suddenly sneezes when he enters an area with (fake) snow flying around; the second sees Jenna Coleman in "The Zygon Invasion" let off an inaudible but still obvious sneeze while standing on an airport tarmac. In both cases, the actors continue with the scene without missing a beat.
- The crew designing Thirteen's TARDIS asked Jodie Whittaker her favorite biscuit, without telling her why. They then put a dispenser with her answer, custard creams, into the control panel, with her only finding out when one dropped out while she was filming her first time in the TARDIS. Her genuine reaction was captured on camera.
- "Resolution": The colourful scarf the Doctor wears was actually a gift to production designer Arwel Wyn Davies from his wife. When the costume designer saw it, he thought it was perfect for the Doctor, and Jodie Whittaker agreed, leading to the scarf being saved for the winter special.
- "Fugitive of the Judoon": John Barrowman was allowed to make whatever alterations to his dialogue that he wanted, like "He's my favourite!" about Ryan. He also held Jack's kiss with Graham longer than intended.
- "The Church on Ruby Road": Ncuti Gatwa and Millie Gibson improvised their banter during the scene where Ruby has to explain who the Doctor is.
