Some of Saturday Night Live's most memorable moments are either this trope or Hilarious Outtakes that turn into Throw It In after much cracking up and struggling to keep composure. Of course, since it's a live show, if the moment happens for the first time during broadcast, Throw It In is the only option.
One of the earliest, and one of the most famous: during Chris Farley's "Matt Foley" sketch, guest star Christina Applegate and cast member David Spade can be seen trying (and failing miserably) not to laugh while Farley goads them on with his famous "I LIVE IN A VAN DOWN BY THE RIVER!" line. The confusion and smirking on Phil Hartman and Jan Hooks' faces are genuine, and the camera cuts to the cast members repeatedly laughing out loud.
Also thrown in was Chris Farley falling and smashing the living room table. That originally wasn't supposed to happen, but it was kept in because it was funny.
Jay Mohr also relates an anecdote of playing a teenager opposite Farley's Matt Foley and being unable to contain his laughter. After failing to hide it, he opted to incorporate it into his performance, rationalizing that a teenager would also probably find Foley pretty funny.
A lot of Chris Farley's most memorable moments on the show were Throw It In! moments that stemmed from something bad happening on the set. When he played Weekend Update correspondent Bennett Brauer who used air quotes all the time, he suddenly found himself flying over the Weekend Update set (or, rather, about to, but ended up getting tangled in one of the stage lights). Farley ad-libs, "I have a weight problem. Can't they lift me?"
When Bill Hader couldn't stop cracking up during his lines as Stefon, he started covering his mouth with his hands. Originally, it was just supposed to be a character tic (since part of the inspiration for Stefon came from a barista that Bill Hader knew who dressed in Ed Hardy, had his hair cut like that, and often had his hands to his face as some sort of nervous habit).
The "Stefon" segments on "Weekend Update" are full of Throw It In!, in the sense that John Mulaney (one of the writers) throws in a lot of changed jokes minutes before Hader's cue, often leading him to crack.
The Debbie Downer sketches became recurring due to the popularity of the first sketch which involved the entire cast (including host Lindsay Lohan) trying desperately to keep from laughing at Rachel Dratch's performance (as well as some misplaced sound effects).
This often happened on Conan O'Brien's Late Night as well. Whenever things went wrong (special effects not working, problems with costumes), Conan would often declare the screw-up to be better that what had originally been planned. In one particularly funny incident, a fire alarm went off in the middle of taping the show. Once it was established that there was no emergency, Conan decided to abandon the planned bit and air the resulting debacle. In another, when a technical glitch stalled a bit he was doing, Conan jumped up on his desk and began performing a striptease in order to pass the time until the problem was fixed.
A bizarre moment occurred during an interview with James Spader which was interrupted when a recording of a voice actor stating "Now that's a good Friday", apparently intended for a skit, was accidentally piped into the studio, leading to some good-natured ribbing of the production team.
To celebrate 20 years as a late night host, Conan put together a blooper reel of the last two decades. Not to spoil it, but the blooper reel experiences technical difficulties. Conan insists on leaving in the resulting footage of the crew trying to fix the problem.
Similarly, Craig Ferguson has left in a lot of bloopers. For instance, he's had the lights go out on him twice (once due to a power failure), and on another occasion, he slapped his Teleprompter too hard and shattered it.
The Game ShowSuper Password was almost ridiculously prone to set breakdowns, most of which were not edited out of the broadcast (for instance, the door sticking, the whole puzzleboard accidentally being revealed, etc.). To say nothing of Convy's utter inability to keep his mouth shut, which often led to him blurting out the puzzle answer prematurely and therefore leading to the whole round being scrapped.
An episode of The Jamie Foxx Show features Mark Curry guest starring as a traffic school instructor who takes his job far too seriously, reaching borderline drill sergeant levels, and eventually breaking into full-fledged military maniac troop leader. While one can't be sure, many of his gags seem improvised, such as one in which he walks into the classroom, trips, and stands up quickly, proclaiming "Any of y'all laugh, you ain't gonna graduate!" and another in which he slips while running towards Braxton's desk before loudly telling him what a "square" he is. In both instances, a number of the cast members couldn't help but laugh, including Jamie Foxx himself, and were forced to turn their heads away from the camera to conceal their laughter.
Listen to how Gibby delivers his line: 'Uh-oh! Looks like trouble off the stern... port... bow.' You'll notice how he seems confused and slows down at the end of that line. The reason? Noah ("Gibby") forgot the line as he was saying it! He couldn't remember how it ended. Later, when I watched it in editing, I felt it played really funny, so I used the take where he forgot the line.
In Buffy the Vampire Slayer, Giles walks into a tree when Buffy reveals that she knows he had sex with her mother under the effects of magical chocolate. This was unrehearsed and done at the last minute, to comical effect.
The first half of season eight, aka Buffy the Vampire Slayer, had motion comics made as well. In the motion comic for Anywhere But Here one line is added for Kennedy. That one line where she makes disparaging remarks about Tara when Willow gets upset about her murder serves to justify every single complaint of her being The Scrappy, which she has become a In-Universe example of.
Fox and NBC's live musical productions (to date: The Sound of Music, Peter Pan Live, The Wiz, Grease, Hairspray, A Christmas Story, and Jesus Christ Superstar) all begin rehearsal several weeks before going to air, using the script as a template. The production team tweaks things as time goes by, such as the characters' actions and lines, based on what they want to contribute, in addition to trial and error. That's why there are sometimes inconsistencies on the shows' soundtrack albums, for which some of the audio is recorded fairly early on during the rehearsal period.
On the commentary for the season 2 DVD of Scrubs, the creator comments that scenes written for the Janitor often had the addendum "or whatever Neil decides to say," due to his frequent habit of improvising usable material.
In John Ritter's guest spot, on Scrubs, he improvised the line "I pooed a little" after his pull my finger gag. Zach Braff immediately had to bite down on his cheeks to keep from laughing.
In the "STOP FINISHING MY AWESOME JOKES!" scene, J.D.'s "Oh, my God" while holding his ears was unscripted — he actually didn't expect Sarah Chalke's voice to get as high-pitched as it did.
The puppet characters occasionally malfunctioned; Tom Servo's head, in particular, would often fall off during production. Sometimes this worked well enough to keep; for example, in a sketch where Tom and Crow were Secret Service agents getting increasingly worked up about protecting Mike, Tom's bubble jarred loose and fell off at the climax, leading them to shout "HEAD! AHHHHHHH!" and panic.
Crow:[Having knocked off Tom's head during a martial arts duel] Uh-oh...I broke him.
Another skit involved Servo blasting Crow with his 'peaceful' death ray. When Crow comes from off-screen, he's quite damaged and has a small fire in his head-net, and the gang have a good laugh about it. However, it turns out that ping-pong balls, such as the ones Crow's eyes were made out of, were rather flammable, and the fire quickly escalated, and everything from Crow's screams to Servo and Mike's increasingly nervous laughter are legit. The edit cuts off before the actors can break character:
Crow (as his puppeteer, Trace Beaulieu): "Ah well, I'll be in my trailer..."
Perhaps the greatest example of this was during the riff filming for a first season episode, when Josh Weinstein, still performing Servo at the time, sneezed violently. Nobody missed a beat.
According to some accounts, Joel Hodgson was really tired during the filming of the first KTMA episode because he had stayed up late the night before building the set and the robots which caused Joel Robinson to have a dopey laid-back demeanor. The crew decided to keep this as a character trait for Joel. Other accounts, however, state that Hodgson had used the same laid-back character performance in his prop-comic standup long before MST3K as a way of coping with his stage fright.
John Cleese: Penguins don't come from next door, they come from the Antarctic! Graham Chapman:BURMA! John Cleese:(looks around the room nervously, a genuine reaction) ... Why'd you say "Burma"? Graham Chapman: I panicked.
The line "intercourse the penguin" was also thrown in by Graham Chapman. You can definitively tell that John Cleese wasn't expecting it and he nearly corpses. In the final version the scene actually cuts to another camera after a few moments and Terry Jones, sitting in front of another camera elsewhere in the studio with his image transmitted to the TV set in the skit, stumbles over his words for a moment and nearly corpses too, suggesting he was caught off-guard, too).
Not an on-screen example, but Aaron Sorkin once ran across Allison Janney entertaining the cast and crew of The West Wing by lip-syncing to a jazz piece by Ronny Jordan called "The Jackal". He liked it so much he had Janney's character, CJ, do it in an episode, and had the characters imply that it was a ritual at the White House's victory parties.
Columbo's famous And Another Thing... mannerism was a sort of Throw It In!. While writing the play Prescription: Murder, which introduced the character, writers Levinson and Link had just written Columbo's exit from a scene when they realized they had forgotten to include an important plot point. Rather than retyping the whole scene, they simply had Columbo come back in and ask "one more thing".
In the original scripts for The Addams Family, Lurch was a mute — but Ted Cassidy ad-libbed his Catch-Phrase "You rang?" in his basso profondo voice while filming the pilot, and the producers liked it so much they gave Lurch a voice, too.
William Hartnell's habit of flubbing his lines was left in due to limited budget. (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.)
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 Expanded Universe 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 Internet Backdraft 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 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).
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 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.
"The Caves of Androzani" features several scenes of the villain Morgus turning to the camera and making Aside Comments. This 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.
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". First take was an accident but the producers loved it so they told Matt Smith to rip it off in subsequent takes.
Matt Smith 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.
In the Seinfeld episode "The Parking Garage", the characters spend almost the entire episode trying to find their parked car in a parking garage. The original ending was for them to find it at the end, drive off, but then be unable to find the exit to the mall. However, when filming the scene where they were supposed to drive off, the car wouldn't start up. Deciding that was funnier, they used that as the ending instead. If you look closely before they cut away to a long shot, you can see Julia Louis-Dreyfus and Jason Alexander shaking with laughter as the car refuses to start.
At the end of "The Subway", Elaine is cranky after missing the lesbian wedding she was best man at. Jerry says "So, you missed the wedding. You'll catch the bris!", and Elaine tries to stare at him furiously, for a moment, before her expression dissolves into a resentful grin. This take was kept in.
In another episode, Jason Alexander sneezes just as Jerry tells him, "I blamed it on you." They felt it was funnier that way and kept it.
In "The Junior Mint", Jerry's line "We'll watch them slice this fat bastard up" was ad-libbed. You can see the actors holding back laughs when he says this.
Kramer's now-iconic sudden entrances into Jerry's apartment began when Richards was late for a cue during one of the first shows, and entered quickly through the door. The audience got such a kick out of it that he kept entering that way in subsequent episodes. It has become one of the character's most lasting trademarks.
According to Jason Alexander in the DVD special features, Jerry Stiller's oddly paced line deliveries were due to him actually forgetting his line for a second, then continuing right when the words came back into his head. Plus, one where he completely gave up made it into the episode "The Strike":
You couldn't smooth a silk sheet if you had a hot date with a babe...I lost my train of thought.
In an exception to the comedy rule, the last episode of the third season of M*A*S*H had, at the end, the characters having a moment of silence upon learning that their former commanding officer Henry Blake's plane was shot down, with no survivors. At the very end of the scene, someone drops a scalpel, shaking everyone from their thoughts; this was unscripted and left in.
The broadcast version does not have the tray dropping; the scene fades out on Hawkeye and Trapper continuing their surgery without pause.
Several minor flubs were left in the show over the years, most of which the actors were able to work around, like an extra who gave his real name instead of his character's name when asked (the actor playing Frank simply took the name he was given and finished the scene without even blinking at the script change).
In Star Trek: Voyager, non-Trekkie Robert Picardo ad-libbed "I'm a doctor, not a night light," in his audition for the role of The Doctor while having no conscious idea that "I'm a doctor, not a [something]" was Dr. McCoy's famous Catch-Phrase. The particular line didn't make it into an episode, but was liked enough that The Doctor went on to use "I'm a doctor, not a [something]" phrases as much as McCoy had, and other Trek doctors would use it on rare occasion as well.
The concept of Jaffa performing a meditation called "kel'no'reem" apparently was inspired by Christopher Judge falling asleep on set during one take, and Michael Shanks quipping "Oh, he's not sleeping, he's meditating."
In season one's "Solitudes" RDA and Tapping ad-libbed a joke that actually led to a minor Story Arc between O'Neill and Samantha Carter. Trapped in an Antarctic ice cave, they snuggle together for warmth.
Farscape apparently had quite a bit of this going on, starting at around about the time Ben Browder ended a scene of armed guards chasing him by whirling around to face them, putting his gun to his own head, and shouting "Nobody move or the white boy gets it!"
In Gigi Edgeley's first appearance as Chiana, the character wasn't intended to stay on the show so she was given painful contact lenses that severely restricted her vision. When it was decided to make her a main character the lenses were modified to be more comfortable, but all the exaggerated head movements Edgeley had done to be able to see what she was doing were kept in as a character trait.
Another example happened in the third season episode "Fractures". Crais and Jool are taking cover behind a med-lab table from someone shooting at them, when suddenly parts of the (still alive and conscious) alien they had been trying to reassemble got blown off the table and landed in their laps. Although it was scripted that only Tammy Macintosh, who played Jool, was supposed to scream at this, she convinced Crais actor Lani Tupu to scream along with her despite it being out of character for the veteran combat commander. The resulting take was considered so hilarious that it was the one used in the episode.
There's also the episode "Meltdown", during a scene in which Aeryn and John are romantically entwined while still fully dressed in their leather Peacekeeper uniforms. At one point there's an almighty creaking noise as leather meets leather, and you can see Claudia Black struggle to keep a straight face.
The cast, starting with Ben Browder, frequently ad-libbed physical contact with the puppets (mostly beating up Rygel), because it made the puppets more believable as real characters if the actors interacted with them physically. The folks from the Jim Henson company were understandably concerned about actors breaking very expensive animatronics.
And in another sketch about a boxing match between the letter combinations "ea" and "ee", Cosby is handed a trophy... which falls apart in his hands accidentally. Skip Hinnant ad-libs: "Sorry we couldn't have sprung for a better trophy." Morgan Freeman cracks up again.
The Red Dwarf episode "D.N.A." has a scene where Rimmer, planning to clone himself to come back to life, is examining some of his own dandruff under a microscope while the Cat looks over his shoulder. During a rehearsal, Danny John-Jules accidentally sneezed on the microscope at the end of the scene. The timing was so perfect that the writers put it in the episode.
In another moment courtesy of Danny John-Jules, the episode "Bodyswap" features Lister's brain being recorded onto a cassette tape and given to the Cat for safekeeping. Almost immediately, he accidentally drops it in his mug of coffee.
From "Quarantine", when Lister, Kryten and Cat are using the luck virus to locate what they need to fight Rimmer and Mr. Flibble, Craig Charles collides with and almost trips over the second item. The DVD commentary suggests that wasn't intentional.
Craig again, from "Holoship". Lister eating the cigarette at the end of the confrontation with a hologram wasn't part of the episode script. Apparently he was sick after the scene.
For Holly's return to the cast in "Nanarchy", he had to give a complicated explanation of the episode's events. According to a later interview, Norman Lovett couldn't pronounce a particularly difficult word in the script, which was Holly-ish enough to make it into the episode as 'that word I can't say'.
In the Supernatural episode "In the Beginning" Samuel is being possessed by the YED and after Dean unwittingly tells him everything, he pins Dean to the wall and smells his neck while asking him if he's one of the "special children." According to Mitch Pileggi, the neck-smelling wasn't rehearsed beforehand so Dean's squicked-out reaction is completely genuine and the sexual subtext is even more creepy.
Another example is Misha Collins' first appearance as Castiel. The staring and ignoring Dean's personal space was not scripted, so that Jensen Ackles was genuinely surprised.
In a form of inversion, Yes, Minister star Paul Eddington had a number of lines cut after he demonstrated that he was capable of expressing everything of significance in the discussion with expressions alone, particularly in the later episodes.
While most of Whose Line Is It Anyway? is improvised to begin with, the 100th episode had a notable incident where the whole song suddenly speeds up right in the middle. Hilarity Ensues. They redid the game with the intention of throwing out the first take, but in the end, the second take was thrown out and the first take was used. It featured Wayne Brady trying to spell out the subject's name "Howard" in rhythm, and he ended up spelling out H-O-R-W-A-R-D. Drew gave him a hard time about it, leading to Wayne responding "It's hard to spell at 210 beats per minute."
Greg Proops, afterwards:
"Watch out for those tempo changes, man, 'cause once we go into the second bridge, this shit takes off!"
Ryan: "Well, I used to be, I can't remember anymore."
The hilarity continued after the game ended. One part included Ryan saying that there "better be some extra points in this", with Drew responding that they "spent all of the points replacing the glass". The second comes after Colin confirms that the glass tubes were lit, claiming that "It would have been even funnier if your head burst into flames."
And Ryan even went on to lampshade the whole thing in his part of the Hoedown song: "But can they break a neon sign by ramming it with their head?"
Should any of the performers, Drew especially, say or do anything while between games that is hilarious, it will become a running gag for the remainder of the show.
One of the best known is when Drew flubbed a line from a scripted card, introducing "African Chant", and stated that "Africa is a country". Greg Proops, the fourth chair that episode, busted up and immediately corrected him that, "It's also a big continent if you're a geographer!" Drew didn't live it down for the rest of the show.
One had Drew using a tape recorder of himself saying "1000 Points!" in an effort to do as little as possible. This provided two instances. One was pointing out the idiocy of making himself obsolete, especially as he needed to rewind the tape. Another was Wayne Brady editing the tape between segments to say "My ass! my ass! My ass! My ass!"
An audience member got one: When asked for a pair of unlikely roommates, a member loudly yelled out "Bill Cosby and Hitler." Drew liked it, but a producer nixes the idea from airing before the game starts. The rest of the episode has the performers made some none-too-subtle jabs at the producer, including Ryan's last line in the hoedown: "He should grow a mustache and move to Germany!"
After one playing of Scenes From a Hat, Drew threw the hat like a Frisbee as usual, but the hat hit a camera and knocked it over. Not only was this kept in the final edit, they even used the footage recorded by the camera as it fell over.
In an early season two episode of House, House is mad at Wilson for having sided with Stacey, and Hugh Laurie ad-libbed "Bros before hos, man" during one take. You can clearly see Robert Sean Leonard cracking up at the line.
The famous "foot in the face" scene in the Merlin episode The Moment of Truth was at least partly just a joke Bradley James came up with — he deliberately waited for Colin Morgan's take, and then... stuck his bare foot in his face. The disgusted reaction is completely genuine, and they did a take without it, though thankfully that's not what ended up in the episode.
The Sifl and Olly Show often had these moments whenever one of the duo would laugh. These moments were usually caused by Liam Lynch. An example can be seen in the music video for "Baby for Gravy". These were also very common in segments featuring the character Chester.
During the twin fight in Skins, Kathryn Prescott and Giles Thomas were supposed to just make it look like Emily hit Doug; however, Kathryn got a little too into the scene and inadvertently lamped him. Giles was so gobsmacked that, in his subsequent confusion and surprise, he trod on Megan Prescott (Katie)'s foot, who spent the rest of the scene desperately trying not to cry. All of that made it to the final cut.
The ending of the famous Oilz scene was reportedly the result of Kathryn Prescott and Lily Loveless just going nuts.
NCIS: According to Mark Harmon, the first-ever Gibbs Slap wasn't scripted; he just broke off in the middle of a scene to whack Michael Weatherly on the back of the head. Not only was it not cut, it rapidly became one of the character's trademarks.
Also, in "The Voyeur's Web", the scene where Abby pretends to cut her throat with the play knife was spontaneous. Pauley Perrette apparently forgot that Gibbs was supposed to be ignoring her and did it as a joke. It was left in.
In the first Christmas episode of That '70s Show, Kelso (Ashton Kutcher) spots Laurie, runs towards her, tries to jump over the couch, but slips on it and hurts his legs with the table, almost knocking over the punch bowl that was on it. Kutcher however manages to keep the straight face and to continue the scene afterwards. This shot was kept, and Kelso's hurting himself later became his trademark.
Behind-the-scenes materials indicate that Kutcher usually tried to roll with goof-ups - like his popsicle breaking in one episode - as just part of Kelso's clumsiness
In the episode where Donna tries to convince Jackie to move in with her, she at one point says: "Jackie, help me be more like you" and she sounds weird when she says it (like if she is running out of breath, or trying to hold a burp, or both). After a few more unsuccessful shots where neither Laura Prepon (Donna) nor Mila Kunis (Jackie) could keep from laughing, the first shot was kept.
Red Foreman's famous threat to shove his foot up X person's ass was ad-libbed by actor Kurtwood Smith and became so popular that it became the character's Catch-Phrase.
The first time in "The Circle", Danny Masterson started cracking up while explaining how the government was hiding engines that ran on water. This wasn't scripted.
Kutcher, meanwhile, completely lost it during that scene and couldn't stop laughing. They left it in and Kelso laughing hysterically during Circles became a Running Gag.
When filming the fifth episode of Glee, when Kristin Chenoweth (April) finishes her first take of "Maybe This Time", Chris Colfer actually cries at her performance. He was surprised to find out that they would use a shot of him crying as his character Kurt's reaction to April's performance.
Speaking of Glee, Brittany's non-sequiturs were all ad-libbed at first, and the confused stares she got from the others were real. And thus her famous Dumb Blonde persona was born. Heather Morris's ad-libs during table readings, often playing on the previous dialog of characters, are sometimes penciled in after the initial readings.
Chris Colfer ad-libbing again: At the beginning of the second episode, when Puck and the other jocks are about to throw him into a dumpster, Kurt glares at them with contempt and says: "One day, you'll all work for me."
More broadly, the character Kurt Hummel can be considered this. Chris Colfer actually auditioned for the role of Artie, but the show's creators were so impressed by his talent and charm that they wrote an entirely new character loosely based on the openly gay actor.
Sam's line "Some people just don't know how to screw things" to Mercedes was ad-libbed. When she laughs, she's laughing for real.
In season 5, after Blaine confronts Elliot about his friendship with Kurt Elliot ends up helping Blaine work through some of his insecurities. At the end of the scene, Elliot suggests they jam out, hands Blaine a guitar, and Blaine proceeds to make up a song called "Glitter Rock Vampire" on the spot (referencing the nickname Blaine had earlier given Elliot). The entirety of the latter half of the scene was improvised by Darren Criss and Adam Lambert.
In season 6, almost all of Kurt and Blaine's dialogue in their trapped in an elevator scene was ad libbed, which made it all seem more natural and organic.
In season 6, when many of the New Directions confront the Tea Party Patriots club, Sam bumps into Quinn and she laughed. This was actually Chord Overstreet bumping into Dianna Agron, and her laugh was genuine.
In one episode of The Nanny, Sylvia fills in for Fran as the nanny. When she sits on Maxwell's desk (one of Fran's trademark moves), the desk collapses under her weight. They decided to keep it because it was funnier than what was originally scripted. If you look carefully, you can see the actors in the scene trying to not laugh.
An episode of Friends dealt with a woman calling Chandler and Joey believing she's calling a guy named Bob, and Chandler picks up, pretends to be Bob, sets up a meeting with her and then shows up to win her over when she's "stood up". The tag scene for that episode had the woman calling again, looking for Bob, this time with Joey hearing the message. The script called for Joey to pick up and say "Bob here", but Matt LeBlanc tripped and fell, desperately trying to grab the phone as he went down. This ended up a lot funnier than the scripted version and was kept for the episode.
In the episode "The One With Phoebe's Uterus", Joey walks into the apartment wearing a blue blazer. Matthew Perry (Chandler), while making a joke, accidently said "black" instead of "back", but the actors' reactions to the mistake were so funny, they decided to put it in the episode:
Joey: Guess what job I just got? Chandler: I don't know, but Donald Trump wants his blue blazer black. (pauses) Ross: What? Chandler: Blue blazer back. He wants it back. Rachel: But, you said "black". Why would he want his blue blazer black? Chandler: Well, you know what I meant. Monica: No, you messed it up. You're stupid. Chandler:(changing the subject) So what job did you get, Joe?
"TOW The Cheap Wedding Dress" has a Deleted Scene (present in the DVD cut) that seems be an ad-lib from Matt LeBlanc: After a love interest ditches both Ross and Joey at a restaurant, Ross asks if Joey is hungry, to which Joey replies, "Does a bear shit in the woods?"
In an episode of Frasier, Kelsey Grammer is supposed to rattle off the line "Fault-finding, flaw-fleeing Frasier" and end the scene by striking a confident pose, but he accidentally says "Flasier" instead. David Hyde Pierce proceeds to make fun of him, still in-character, ("You said 'Flasier'!", with a Niles-esque smug grin), and Grammer responds also in-character, protesting that he did not say "Flasier", since he's been saying his own name for forty-some-odd years. The two then ad-lib talking over each other, tailing off into one of Niles and Frasier's very frequent scene-fade-out squabbles. Coincidentally, Grammer saying "forty-some-odd years" instead of whatever Frasier's exact age was (Grammer probably didn't know it off the top of his head) was completely appropriate in the context of the preceding conversation.
In the season three finale of How I Met Your Mother Barney is in a meeting with a bunch of Japanese men when Lily calls him to tell him that Ted has been in a car accident. He gets up, tells them in Japanese that his best friend needs him and he has to go, and hurries out the door. One of the men then turns to the others and asks, in English, "What did he say?" That line was ad-libbed during rehearsals by one of the extras in the scene, however it was given to one of the others to say in the actual episode.
In "The Final Page", the tears in Ted and Robin's eyes when Ted tells Robin to go after Barney weren't in the script. The scene was so emotional that Josh Radnor and Cobie Smulders couldn't help themselves.
Battlestar Galactica (2003) had a lot of this too... a fair amount of it probably stemmed from Edward James Olmos' Method Acting. The most famous Throw It In! was probably the scene when in a fit of rage over losing someone, Olmos-as-Adama destroyed a model ship that his character had been building. Olmos thought that the model was just a prop, and smashed it in a single infuriated take that made it into the show. Little did he know the model was actually on loan from a maritime museum, and had a value of around $100,000. Fortunately, it was insured.
When Roslin promotes Adama to Admiral, she was supposed to kiss him on the cheek. Olmos kissed her on the lips instead, and it was used in the episode. Mary McDonnell would later go on to say that she believes it is during that scene that Roslin falls in love with Adama, although it would take them another two seasons to get around to admitting it.
Grace Park threw in the lullaby that Boomer hums to the Raider and Athena hums to Hera, which is a traditional Korean lullaby.
Gaeta's savage baiting of Starbuck with the line "I suppose a pity frak is out of the question, then?" at the end of their mess-hall conversation in the episode 'A Disquiet Follows My Soul' was improvised on the first take by Alessandro Juliani. The entire set cracked up; then Ron Moore decided he liked it and asked AJ to keep it in in subsequent takes.
In NUMB3RS David Krumholtz inadvertently misspelled "anomaly" as "anomoly" on a map in one scene in the pilot episode, which the writers and producers kept as a quirk for Charlie. They even reference it in the final episode of the first season:
Charlie: I know how to spell the word "anomaly", okay? Don: No, you don't. Charlie: Get a dictionary. [...] Don:(shows him the dictionary) See? One O. Charlie: Is this a reliable dictionary?
Also referenced by other characters throughout the show. For example, Alan confides in Larry that he hates playing chess with Charlie because he seems so bored, so Larry tells Alan:
"Try Scrabble. He's a horrible speller."
In Defying Gravity Wass's remark that he could "sleep through World War IV" caused a lot of fannish speculation as to whether this was a hint of the state of the world in 2052. When creator James Parriott was asked about it however, he claims not to remember writing the line, and that it must have been a throw in by the actor.
In the final scene of Gossip Girl's second season Chuck was supposed to tell Blair "I love you too" and then, after she kisses him, "I'm not Chuck Bass without you." The second line ended up being moved to the season three premiere, since they instead went with a take where the actors ad-libbed the following:
Blair: But, can you say it twice? No I'm serious, say it twice. Chuck! I love you. I love you. That's three. Here's four — I love you.
According to this page, the first episode of the cop show Matlock Police (not to be confused with the more famous Matlock) had a car chase, where "the more dramatic aspects of it happened by accident — literally." (See link for details.)
In a Sex and the City episode, Miranda is contemplating her "just friends" relationship with on-again, off-again boyfriend Steve (who is the father of their son Brady). In an earlier scene in the episode, Steve picks Miranda's favorite flower, lilacs. Later in the episode, she's at a wedding, and, setting, a sprig of Lilac onto a table, asks her 1-year-old son Brady, whom she's holding, whether she ought to give Steve another chance. A second later Brady stretches out his hand for the flowers and picks them back up. This was unscripted, and worked for the scene particularly well.
In the CSI: NY episode "'Til Death Do We Part", Mac comes in to the lab to talk to Stella. At the end of the scene, someone comes in and gives Stella a crucial piece of evidence, to which she says "Best part of the job" and kisses Mac on the cheek. According to Melina Kanakaredes, who plays Stella, while filming that scene, she just spontaneously kissed Gary Sinise on the cheek and they decided to keep it in.
Just prior to the filming of the pilot for WKRP in Cincinnati, actor Richard Sanders (playing Les Nessman) injured himself, and thus appears in the pilot with a very visible bandage. It became a hallmark of the character that, in every subsequent appearance of the character, he always had some sort of visible bandage somewhere on his person.
David Lynch was famous for incorporating a number of Throw It In! moments into Twin Peaks, including one which introduced a major plot element: during filming of the pilot episode, the face of Frank Silva, one of the set workers, was accidentally caught on camera reflected in a mirror. Lynch liked the effect enough to keep it in, and later cast Silva as BOB, the malevolent spirit who turns out to be tormenting Laura and her father. In addition, a number of improvisations and line-fumbles were kept in, even when they didn't actually make sense in context, in order to enhance the general weirdness of the show.
As much as 90% of Let the Blood Run Free was this, as the scripts were written based on audience votes, so the cast (all veterans of the original improv show) wouldn't see them until days before the episode aired.
Star Trek: The Next Generation had a season one episode, "Symbiosis", where in one of the last scenes Tasha Yar is in the background. Blink and you'll miss it, she waves at the camera. This was the last episode filmed with Denise Crosby as a regular cast member. The next episode, "Skin of Evil", where the character leaves, was filmed before "Symbiosis".
A physical example occurs in Terminator: The Sarah Connor Chronicles pilot episode, where Cameron has a mark across the side of her face where she was damaged in a fight scene. In reality, that injury was real; Summer Glau was hit in the face with an ejected brass casing while at the shooting range. Instead of covering up the mark, Summer decided to integrate it as one of the wounds she'd suffered in combat, and it shows throughout the pilot.
In a later episode, Jesse had to get into Walt's car quickly to get away from the cops, but the door jammed while Aaron Paul was trying to open it. Paul and Cranston reacted in character, and director Vince Gilligan left it in the final cut of the episode.
The show has always excelled at communicating non-verbally while never underestimating the power of words. Holly saying "mama" in "Ozymandias" was not scripted; the baby "playing" Holly saw her mother who was nearby and called for her. The director, Rian Johnson left it in because it expressed so well what Cranston was originally going to sell with a look.
The opening of the MacGyver episode "Deathlock" shows a helicopter very nearly losing control due to ground resonance. However, the pilot averts disaster by lifting off, and the actors seem none the wiser.
When one of the pilots for Wheel of Fortune was filmed, the puzzle board was originally intended to be mechanical and self-revealing, like the rebus board on the old Concentration. There wasn't enough time to finish building it, so the producers just gutted what they had and hired Susan Stafford to turn the letters manually. After the show got picked up, they kept the board as it was. Of course, the letter-turning position was later taken over by the iconic Vanna White. The board was enlarged a year before she took over, and changed to a set of touch-activated monitors in 1997 (which were converted to LCD screens in 2007).
They've left in a lot of glitches over the years, particularly when the board refuses to cooperate. One episode not long after the introduction of the electronic board had Vanna hitting a monitor with her fist before it cooperated.
On April 20, 2012, the lights all went out after the Bonus Round. Surprisingly, for a show with a lot of edits, this was left in.
Speaking of Concentration, the 1980s revival Classic Concentration had large fake palm trees brought in for a themed week. They later became permanent set pieces mainly because the people on the show liked how it looked. More foliage was later added to the set to give it more of a "California" feel.
Similarly, according to the book Odd Couple Mania, there were numerous times on that show where blank script pages are preceded by the phrase "Oscar teaches Felix". Tony Randall and Jack Klugman would then improvise the teaching scene.
In the first episode of Leverage, there is a scene establishing the Unresolved Sexual Tension between Nate and Sophie and Hardison rolls by on an office chair saying "oooooh...." Aldis Hodge wasn't even in the scene but had stuck around to watch.
Leverage is full of this trope.
"The Iceman Job" has Hardison asking Eliot to "hug it out," "it" being the conflict brought about by Hardison's Big Lipped Alligator grift persona. Christian Kane is clearly surprised by the attempt at hugging.
In "The Rashomon Job", Parker's victory dance/weapon brandish is ad-libbed.
Ditto the hug which Eliot gives Hardison as Hardison stands with his arms open after Parker obliviously walks away from his attempt to hug her.
"High five for morale," after "The Gone Fishin' Job". Word of God states that the actors throw it in once in a while as an indicator of how much the friendship between the two Vitriolic Best Friends has evolved.
Leverage actor Aldis Hodge also goes by the credo used by Robin Williams and Nathan Lane in The Birdcage, he'll do one take straight and then "give 'em hell" the rest of the takes.
The studio audience for the All in the Family episode "Edith's 50th Birthday," in which Edith is nearly raped by an intruder, became so enraged at watching a beloved character under attack that when Edith hits her attacker with a cake pan and runs for the front door, a number of women in the audience can be heard screaming "RUN!"
The exceptionally powerful audience response is explained by the director, Paul Bogart, choosing to tape the entire sequence straight through, without a break, allowing the tension to continue mounting until her thrilling escape provided them with well-needed catharsis. Bogart explains his rationale in this interview segment. The episode would win Bogart that year's Emmy Award for Outstanding Directing in a Comedy Series and remains one of the greatest arguments for a live Studio Audience over a Laugh Track.
David Dukes, who played Edith's would-be rapist, stated that the audience grew so hostile during the filming of this episode, he was afraid that audience members would rush onto the set and attack him.
In the pilot, Tobias Fünke auditions for the Community Theater of Orange by singing "I'm Bad, Bad Man" from Annie Get Your Gun. Actor David Cross accidentally stuttered during one take. The stutter take was used in the episode.
The scene in episode "Motherboy XXX" shows Buster accidentally slamming the door on Michael's face as they go after Lucille. This was unplanned, but the director liked Jason Bateman's reaction so much that it was allowed in the final cut.
Later Tobias, in disguise as a Mrs. Doubtfireexpy, tries to make a Mary Poppins-like stunt with an umbrella to entertain Maeby. It results in David Cross loosing his balance, crashing into a coffee table, and smashing it. Amazingly, Cross, despite clearly being in actual pain, manages to stay in-character, and, even more amazingly, Alia Shawkat, aside from making a quick Aside Glance, manages to keep a somewhat straight face throughout the whole thing.
The cast of The Office (US) often improvises, much to the surprise of the staff. In one episode, Michael, wanting to prove to Oscar that he's not homophobic, forces himself to kiss him. The awkwardness was not scripted and the actor's reactions seen were genuine.
Another episode shows a Cold Opening where Jim wants to annoy Dwight by popping his balloon chair. The script was hoping that the balloon would deflate but the sudden popping and the surprised reactions were thought to be funnier.
On the Colgate Comedy Hour anything funny which came up during pre-show rehearsal would likely be written into the script.
During a sketch in one episode hosted by Martin and Lewis, a prop towel dispenser fell open when Jerry Lewis yanked a towel from it. After several failed attempts to close it, Lewis broke character, walking off-stage and dragging the man responsible for the prop onto the set, and made him stand there, holding the cover of the dispenser in place for the rest of the sketch.
On the British Panel GameJoker's Wild, chairman Barry Cryer quite frequently made small slip-ups regarding the amount of points awarded, who was speaking at the moment, the subject and so on. This could have passed quite unnoticed, except the panel kept insisting on calling attention to these mistakes...he had two catchphrases to lampshade this: "I only said that to see if you were paying attention" and "Tune in next week; I won't be here."
Burton Richardson announced the first episode of the game show Greed until it was decided to replace him with Mark Thompson. When announcing the first episode, Burton accidentally called a contestant "Michelle Smith" instead of Michael. Mark Thompson "re-created" this blooper when he was chosen as the series' announcer (all his work was done in post).
In an episode of The Goodies, Graeme and Bill have just had a fight with Tim, and attempt to leave the room in a huff...but break down crying on the way out. Tim loudly mentions how he could use someone to work for him, at which point they both barrel back into the room. Fairly ordinary stuff...except Graeme slipped on the carpet while running back in, slid momentarily, and grabbed onto Tim to keep from falling over, nearly knocking him down on the process. They kept on with the scene as scripted, and it was kept in the final cut — you can see Bill covering his face to stop himself giggling.
The hidden-camera show Impractical Jokers had this happen with Q when his tooth fell out on a speed-date, and the guys simply challenged him to act like a pirate.
Psych often includes ad-libs. The most notable of which is the long-running gag of Shawn introducing Gus using absurd, fake names — sometimes resulting in Awesome Mc Cool Name or Unfortunate Names. The first use was a surprise ad lib by James Roday. Eventually, the names became written into the script.
On The Cosby Show , Bill Cosby often kept the camera rolling when his many child co-stars would make mistakes because he felt it was funnier that way. In general, there was a lot of ad-libbing that made it into the final cut of each episode. Examples include:
Rudy's friend Peter was never supposed to be silent. The child actor froze once he was on camera and Bill took the opportunity to turn the character into the Silent Bob.
Then there was the episode where he was taking care of Sondra and Elvin's twins. They would continually look up at the obvious stage lights. Bill played off on it as if the babies kept staring into space. One of the babies spit-up in the very same episode but Bill cleaned the baby up and kept going.
One episode was to just show Cliff making a meal, and Cosby turned it into an extended bit lampooning Julia Child.
Early seasons had Rudy forgetting her lines, so Bill Cosby just spoke them for her and kept going.
One actor continually rendered "Doctor Huxtable" as "Dostor Husstable". It simply became a trait of his character.
The Golden Girls: In one episode, Rue McClanahan and Bea Arthur break character and begin laughing on camera as Betty White delivers a funny monologue about a "herring war". According to interviews with the cast, this was actually a taped rehearsal for the scene, but the producers liked the spontaneous reaction so much, it became the final version of the scene.
One episode features Mrs Doyle recites a long list of profanities which she has read in a steamy novel, supposedly to demonstrate how offensive she found it. The last one, just as Ted ushers her out of the room and slams the door, was "'Ride me sideways!' - that was another one!" That was an ad-lib by Pauline McLynn, which was left in, though the scene had to be ended immediately afterwards as Dermot Morgan (as Ted) promptly burst out laughing.
Another episode had Father Dougal giving a number of increasingly ridiculous descriptions of the "Beast of Craggy Island," culminating in the Logic Bomb claim of "instead of a mouth, it's got two faces." Ardal O'Hanlon didn't think this was funny, and so during filming changed it to "instead of a mouth it's got four arses," which caused the studio audience to laugh for so long that Dermot Morgan had to wait nearly a minute to continue, and the writers had to concede that, low-brow as it was, O'Hanlon's version was the funnier one.
At least twice on Highlander. In 'Money No Object', Duncan and Amanda were supposed to kiss, but the actors' timing was off and they bumped heads. The episode was a comedy, though, and the mistake flowed well with the rest of the ep, so it was left in. Another was a season 2 episode ending where Adrian Paul and Stan Kirsch expected the director to cut the scene earlier than he did. They start laughing after a few moments, wondering what's wrong, and the cut finally came, but the unexpected part was left in.
For example, in "Mr. Monk and the Leper," the scene where Randy goes to Dr. Polanski's office to take down some pretty embarrassing photos depicting him with severe acne, having been told of their existence by Natalie. He tries to take one down, but it turns into an epic struggle, and when the photo does come off, it takes part of the wall plaster with it...just as Dr. Polanski walks into the room. According to sources, Randy was only supposed to do a simple swipe, but the set designer nailed on the picture so well that it took several tries and some loosening of the original photo for Jason Gray-Stanford to get the photo down.
In an episode of The State, David Wain forgot half the steps for his dance solo in "Porcupine Racetrack" and then he fell down. They kept it.
In Strangers with Candy, typos in the script were often left in because they sounded funnier, resulting in characters saying phrases such as, "I'll be back in a shortly."
In Stranger Things, Steve was originally intended to be more of a traditional Jerk Jock character; he was supposed to try and force himself on Nancy, for example. Joe Keery played him too nice for that, though, so they decided to make him more nuanced.
Parks and Recreation makes use of varying degrees of improvisation. The most frequent example are the "addressing the camera" scenes with many jump cuts between jokes; these are usually the result of letting the actors run off-script (especially with Amy Poehler).
In the episode "Article Two", when a Pawnee citizen named Garth (Patton Oswalt) filibusters a city council meeting by rambling about his thoughts for the new J. J. Abrams-directed Star Wars (which ends with it becoming a crossover with the Marvel Cinematic Universe), the filibuster was entirely ad-libbed by Oswalt. He in fact rambled for 8 minutes, but most of it was cut.
In "Flu Season", Andy looks up Leslie's symptoms and says, "Leslie, I typed your symptoms into the thing up here and it says that you might have...Network Connectivity Problems." This line was ad-libbed by Chris Pratt, and Mike Schur, the series' co-creator called it his favorite joke in the whole series.
In "Harvest Festival," there is a part where Ron slips and falls down while running. Nick Offerman actually accidentally fell down at that part.
Leslie's horrible attempts to sing along with "We Didn't Start The Fire" in the episode "Leslie and Ron" was completely improvised by Amy Poehler.
The character of April Ludgate did not exist in the original conception of the show. The role was written specifically for Aubrey Plaza, leading to some of the funniest moments of the series. Allison Jones, the casting director, met Plaza, and the actress made her so uncomfortable that Jones contacted the creators to insist that they cast "the weirdest girl I've ever met in my life." The role of the creepy and apathetic intern was written into the pilot script, originally simply named "Aubrey".
There is a hidden blooper of sorts in the Ultraman Zero Gaiden: "Ultraman Zero vs. Darklops Zero". If you look closely during the final battle between the Zeroes, as well as Gomora and Mecha Gomora, Gomora kicks Mecha Gomora in the head and one of the robot's head crests falls off. Because they thought the scene was obscured enough by Ultraman Zero's and Darklops's battle, the blooper was left it.
Reno 911! shot its station scenes at a real police station. In the second season finale, recurring character Terry is arrested, but manages to escape to the parking lot. When they shot the escape, they ran out to the parking lot to discover a helicopter taking off. All the actors had to do was pretend Terry was on it.
Babylon 5: A particularly interesting form of ad lib occurred in the third-season episode "Convictions". There is a scene where G'Kar and Londo (who hate each other) are trapped in a lift car. The original script called for G'Kar to speak to Londo about his lack of sympathy for their situation in a cold and heartless tone. But for whatever reason, Andreas Katsulas (who plays G'Kar) delivered his lines in a completely different tone: that of Comedic Sociopathy, laughing like a madman at times. It completely puzzled J. Michael Straczynski at the time, but once he realized what happened, he realized that Katsulas had outdone the script. Not only did he throw it in (a rare event indeed), but he remembers it as one of his favorite moments of the series.
The Firefly episode "Shindig" called for Mal and Inara to dance, and for Mal to suck at it. However, Nathan Fillion and Morena Baccarin spent so much time practicing the scene that Fillion learned it quite well and couldn't convincingly play someone who couldn't dance. They left that take and dubbed in a line by Mal saying "This dance I think I actually know."
Fawlty Towers mostly kept to the script, although there was one case where Basil (John Cleese) and Manuel (Andrew Sachs) are busy talking about a rat, with Manuel denying that it is a hamster. Normally, he speaks in broken English, but in one instance mutters, in perfect English, "It's not a rat, it's a hamster!". The shock on John Cleese's face is priceless, and as a result it's become rather memetic.
In the Castle episode "The Final Nail", Beckett interrogates a man with a thick Russian accent. During the interrogation, she ends up mimicking his speech pattern, rolling the 'r' in "cry" before catching herself. Stana Katic does speak fluent Russian and her slip was purely unintentional, but it added levity to the scene so they kept it in.
Many of the bloopers in Hee Haw were funnier than the actual jokes, so they just left the camera rolling.
In this promo for a bakery in a French TV show, a running cat bumps into an open door.
Star Trek: The Original Series episode "Amok Time". After Captain Kirk apparently dies during a duel with Spock, Spock has an emotional reaction when he learns Kirk is alive. When he denies doing so, Dr. McCoy says "In a pig's eye!". According to the science fiction author Theodore Sturgeon, who wrote the script for "Amok Time", the line was ad-libbed by De Forest Kelly.
On Lost, the character of Sawyer was supposed to be a slick, suit-wearing city con-man. However, during his audition, Josh Holloway (who ended up with the part) forgot his line and so kicked a chair in frustration while swearing profusely. The producers liked the edge he gave, and so the character was rewritten to be a darker Southern grifter.
Kari Lizer's Vanity Plate, Kari's Logo Here, wasn't intended to be used as the actual end logo for The New Adventures of Old Christine—a technician made it as temporary filler to drop once they actually made one. Kari liked the design of it and decided to use it as her actual end plate instead.
"There's no 'mistakes' in painting, just happy accidents."
In one episode of Blackadder Goes Forth, Blackadder mentions that only two of the following three universities - Oxford, Cambridge, and Hull - are truly great. Stephen Fry, who attended Cambridge, while Rowan Atkinson attended Oxford, made a completely improvised jab at his alma mater's rival by saying "Indeed! Oxford's a complete dump!", catching Atkinson off-guard.
An In-Universe example from Murder, She Wrote - while writing a script for a video game the game keeps crashing when the player tries to speak to a specific character. Because it would be too difficult to debug the character has to be removed, except that he has crucial information related to the game's story. A programmer suggests that Jessica change the script so that another character give the same information in order to keep the information relevant.
The pilot script for Steven Moffat's sitcom Joking Apart contained a typo that gave the character of Tracy the line "You're a silly". The actress delivered the line as written, and everyone realised that, since Tracy was very much The Ditz, it suited her perfectly. Not only was it kept in, it became her catchphrase.
In Ressha Sentai Toqger during an episode featuring revived monsters, Tokatti accidentally bumps into Mio, who can be seen and heard apologizing to her castmate.
The core cast of Silicon Valley are all seasoned improv comedians, so this comes up a lot. A few examples:
In the pilot, Erlich claims to own, "a small percentage of Grindr". This line was improved by actor T.J. Miller. Originally he said 10%, but was asked to reshoot the line when they realized that would make him a multimillionaire.
In the season one finale, Jared pitches a variety of different ways that Pied Piper could pivot their business model. The director asked actor Zach Woods a bunch of different ideas, some of which made it into the final cut.
Likewise, for the scene where Erlich and "Action Jack" meet for the first time, T.J. Miller hurled four and a half minutes worth of insults at Stephen Tobolowsky. The final scene was 20 seconds long.
Richard's reaction to the soon-to-be infamous horse fucking scene ("this is just .. hard for me") was unscripted.
During filming of Robin of Sherwood, there was an accident when the horse ridden by Robert Addie, as The Dragon, Guy of Gisburne, accidentally clashed heads with Jeremy Bulloch as Butt-Monkey Edward of Wickham. The moment was included in the show, reframed as a Kick the Dog moment with Guy deliberately making the horse head-butt him.
The Avengers, in its early seasons, was virtually performed live, with no time allotted for retakes. As a result many flubs and errors were kept in. Some were surprising, such as one early Honor Blackman episode in which a camera crashes into a piece of scenery mid-scene, making it appear - and sound - as if a bomb had gone off in the studio, and in another episode, actress Julie Stevens stammered over introducing John Steed to another character, getting his name garbled, leading Patrick Macnee to ad-lib a joke in-character to cover.
Barney Miller: After writers saw Jack Soo eat take-out Chinese food using a pair of pencils as chopsticks, they had his character, Nick Yemana, do the same thing. The bit where Yemana accidentally ate the eraser off the end of a pencil was the writers' invention.
This often happened on Mister Rogers' Neighborhood but Rogers would insist the scenes be left in to show that mistakes can always happen. Incidents included messing up his zipper during the opening, interrupting his song, and another episode where he and a friend try to get the fish in his tank to make noises to no success.
In an even bigger example, one early episode's script had the note "Kuroto laughs at Emu Hojo/Kamen Rider Ex-Aid's despair" with no further elaboration. Kuroto's actor Tetsuya Iwanaga decided to go full-on Laughing Mad, which started the process that completely redefined the character, changing him from a bog-standard Corrupt Corporate Executive into an egomaniac whose God complex is matched only by his hamminess.
Agents of S.H.I.E.L.D.; In "S.O.S. Part 2", Fitz uses his tech to block Gordon's teleportation. When Gordon demands to know how he did this, Fitz answers, "Science, biatch!" The original line was just "Science!" but Iain De Caestecker's ad-lib was kept.
Toward the end of "Bouncing Back", when Simmons and Fitz agreed to put their baggage behind them and give their relationship a second chance, the scene ends with the two re-introducing themselves to each other and shaking hands. As the camera pans away from them, they give each other a fist-bump. The fist-bump wasn't scripted but was so in-character for the two that the producers kept the scene as shot.