Almost every joke in Da Suisa, you know is improvised when the actors laugh.
Machinima as a genre took off largely due to Red vs. Blue — and Red vs. Blue uses a glitch that caused magnums to look like they are held at ease, allowing civil-looking conversation. (Look all the way down with the magnum. The player model's head snaps back up.) Back during the Halo CE sections, it was noticeable when they drew their weapons because they were suddenly looking down for a moment. Bungie liked it so much that it was a feature in Halo 2. (When out of grenades, hold the grenade button.) They scaled it back to only available in local games in Halo 3 and Reach as a move to prevent BXR game-breaking button combo shenanigans. Halo machinima suffered, with many of the large teams who didn't happen to live and work together falling apart.
In Dragon Ball Z Abridged, Kaiser Neko's "Episode Breakdown" videos generally show exactly what all was thrown in on a whim.
In their Dragon Ball ZKai Abridged Episode 2 alone, we have Vegeta coughing up blood during Goku's "You killed Vegeta!" line (because Masako flubbed a take), Freeza flipping everyone off when asking "Who's next?" or even the same arm re-edited for a visual gag when it falls next to Freeza's dying form when he tells Goku that his spaceship is "That way".
Less funny than dramatic, but Brittanny Ashworth's death in V2 of Survival of the Fittest. The character's handler said, in an OOC note at the end of his first post with her, "Anybody after a kill feel free to take a shot at her." Of course, somebody interpreted this a bit too literally, godmoding the character's death at the hands of his villain character. After a little debate, it was left in.
Also, an important part of V3 character's Adam Reeves' backstory revolves around him having his lip torn by a fish-hook hold in a fight. It was suggested the culprit of this injury was the older brother of Maxie Dasai. The more the handlers thought about the idea, the better it sounded, and it was simply thrown in.
The website Bloggingheads.tv has a lot of this. Cats, children, doors, phone calls, and various other interruptions have been known to occur, and are sometimes left in.
This. Part Two was originally meant to be uploaded here, but the camerawoman thought the dissonance would probably be taken as an excellent piece of satire. See also the adverts on the same channel.
JonTron includes many outtake-worthy scenes as part of the show's humor.
While tinkering with Ganon's clothes during the production of The Legend of Zelda: The Light of Courage, Ganon's robe fell off for no reason. The creators decided that the thought of the Great Evil King's robe falling off was too funny to leave out.
According to one of Linkara's video logs on the filming of an episode of The Nostalgia Critic, the Zuul joke in The Pagemaster review was originally a throwaway joke from Doug's brother, and the two decided to keep it in the video anyway. It got to the point of becoming a proper Running Gag in future Critic videos, and Doug and his brother can apparently no longer watch Ghostbusters (1984) without shouting "Zuul, muthafucka! Zuul!"
More dramatically, Critic's rant in the Scooby-Doo review was only meant to be two lines, and Doug improvised a whole Despair Speech.
Marble Hornets has a special-effects example: at the end of entry 22, when Alex bleakly informs the camera how his situation has hit rock bottom, a shadow passes over him in a way that makes it look like it's reaching into his head. The shadow was intentional, the way it ended up looking wasn't. They kept it in.
In Entry #9, Tim's line to Alex about 'not having enough money to pay because he spends it all on tapes' was completely improvised by Tim Sutton (as in the real-life actor) himself. They decided to add the line into the script, but first they had to stop the take and re-shoot it because the original improvisation sent Troy Wagner into a laughing fit.
While building the original AJCO facility for the roleplay sets, A_J's player (the aptly named AJ) accidentally dug through into bedrock, exposing the endless blackness underneath the Minecraft world. She created a lab room around it, and many RPs later the Void is the most important plot point of the whole series.
A_J's distinctive Black Eyes of Evil also came about when Pythosblaze, Frances' player, drew her first fanart of A_J and misinterpreted AJ's badly-shaded skin as having completely black eyes. AJ rolled with it, and it was explained in-RP as one of the side-effects of continuous exposure to the Void.
Similarly, Breyos' and Dell's players both drew Vinnie with pointy ears when they were meant to be a human, and Vinnie's player Cameo decided to keep it and explain it as plastic surgery due to dramatic bullying on their home planet.
The flash series 4SwordsMisadventures had quite a few ad-libs in its episodes:
In episode 4, the priest who expressed shock at Link's swear was not originally supposed to speak spanish, but Haidouken Dude thought it would have been funny.
Red stating that Green should "keep the tunic on" was added in by the creator.
When writing the scene for them running to the elder's house, he unintentionally made a reference to Resident Evil.
During the episodes where Red Link ended up becoming drunk from Chateau Romani, Red's voice actor suggested that he give Red a slurred southern accent while he was drunk, as well as improvise several of his lines (such as when explaining how he "knew" that "Chuck Norris's fist" was the answer to the third question as well as several lines in Red's drunken rant during the second part of the Phoenix Wright-esque debate). Another VA also suggested that they add in Red stumbling while entering the Castle Gates.
In Episode 5, the line where one of the villagers identifies Blue and Purple Link (aka, the Cock Boys) to the crowd was originally intended to be for the Running Man character that they met beforehand, but Haidouken Dude wanted to finish the episode quickly so he rewrote it for another villager.
In Episode 6, Vaati's slide show detailing Tingle ripping off the Links with maps at high prices, as well as Link's "It's over 9000!" line within the slideshow was improvised by Haidouken Dude.
The pair of Hinoxes that Shiek bumped into was originally supposed to both be voiced by Haidouken Dude, but he ended up voicing the green version instead.
Most of the infamous riot scene in the ending of Episode 6 was actually improvised and not in the original script.
Red's reaction to and speech about the potion eliminating his hangover in the ending of Episode 7 was an ad-lib. Likewise, his snoring while passed-out drunk from the aftermath of the fight as well as his earlier line of "Let go of me, you Purple Pissant! Me and the Green boy gotta talk!" in response to Green's statement of he should kill him first if he should get to Zelda, and his line when giving a drunken beatdown on Dark Link ("Oh look, I broke a nail on your face!"), were improvised by Red's voice actor, Richard.
Haidouken Dude even admitted that he often allows for the voice actors to take liberties when delivering the lines and doing situations when making the episode after creating the script.
Most Shiny Objects Videos involve at least a little ad-libbing. The Stinger at the end of the board game segment from "On The Couch" is a good example—the actor Nathan Schneekluth found Catch Phrase so amusing he kept pressing buttons during takes. One of them made it in.
Mr. Mendo’s Hack Attack is created almost entirely this way. The show is co-written by two men, Mendo and Winston O’Boogie, who live on opposite sides of the country. By their own admission, when they write their scripts, nine times out of ten, Mendo will reference a film Winston (the editor) has never heard of (which leads to changes in the script without Mendo’s knowledge) or Winston will call for a prop that Mendo doesn’t have access to (leading him to improvise with whatever props or stock footage he has lying around his house). Since neither one knows what the other is doing until the editing process, each episode is wildly different than what either intended, giving the show an anarchic appeal. The most infamous examples being:
The crossover review of Leprechaun: Back 2 tha Hood originally called for a joke about Mendo eating a salad. At the time of filming, he didn’t have any in his house. He did, however, have an inflatable Charlie Sheen doll…
The American Virgin review, had a running gag of Mendo wearing a completely different outfit every other line, culminating in shot of him in the editing booth blaming the bad continuity on society…while doing cocaine off the edge of a knife. The episode was filmed right after Mendo moved to a different house, and between working out the bugs in his new set-up and getting settled, the shoot went on for two weeks; Mendo decided it wasn’t worth remembering what shirt he was wearing and played the bad continuity up for all it was worth. And as to why the cocaine gag offered a rare glimpse of his eyes? He came up with that line while on a Skype call in a different room of the house and just didn’t feel like going back for his sunglasses.
To make the episode even stranger, Mendo had employed a different editor, who kept adding caption gags. Not to be outdone, Mendo responded by adding in various clips from YouTube. The resulting episode is a fan-favorite!
The "Wild Card" epsiode of Trifecta on PJTV (the one regarding Obama's current vendetta against the SCOTUS regarding their lack of support for the Obamacare bill) was one big Throw It In, as the topic was chosen at random and the answers were done without any prior scripting.
In the Escapist News Network episode Games for Pets, Kathleen was supposed to just run off at the end. However, in her haste, her chair hit the background, which caused the "ENN" logo to break and the "E" to fall off. They left it in because it was just hilarious.
Coyle Command 's Agent B ended up stranded in The Void due to a telportation mix up in the episode He Blinded Me With Science. Originally this was just a one-off gag, but Simon (B's actor) proceeded to film a video depicting the character's escape. The video was held off on release for comedic purposes, until the character ended up trapped for almost an entire year.
Most of Anni and Dr. Pocalypse's dialogue is improvised.
According to the commentary on Dr. Horrible's Sing-Along Blog, when Captain Hammer tries to shoot Dr. Horrible with his own malfunctioning death ray and sneers, "I don't have time for your warnings", it was one of a very few ad-libs in the feature. He also apparently improvised wailing "Why is this fire exit locked?!" on his way out.
Some elements of We Are All Pokémon Trainers were created simply because someone did something random and the rest decided it was good enough to throw in. Examples include:
Jane's inexplicable Southern accent at the beginning of the RP being explained as a Hoennese (In particular Mossdeep) accent.
Champloo, Dune's Lucario, decided to make a batch of super-chocolaty brownies. It was suggested that, do to the sheer endorphin-releasing goodness of that much chocolate, they should be able to reduce a Shadow Pokemon's shadow gauge. This is eventually how Vega the Drilbur purified.
In episode 61 of Freeman's Mind Ross noted there were two instances of this while defending the portal. The voice of the scientist was so quiet that he couldn't be heard where Freeman was standing. While the voice could have been enhanced or dubbed over, Ross found the effect and Freeman yelling at him to speak up funnier. The other instance was when Ross accidentally fell into a pit. Not only did it save the hassle of reshooting the scene but it felt more natural.
Earlier, in episode 42, Freeman wildly fired at the chopper harrassing him and actually shot it down. With an MP5. Ross admitted that wasn't in the script, and the next episode lampshaded the improbability.
In episode 1 of Ten Little Roosters, Michael and Barbara start getting into a No, You shouting match, with Michael ending with "You're sucking eight dicks... from every direction just like an omni-directional dick-suck fest!" It was left in because it was hilarious.
In one episode of Red vs. Blue, Sarge is barking out orders to the Red Team and at one point tells Donut to run around and scream like a little girl. Originally, it was supposed to have been him just screaming a bit as he ran off screen, but he ended up running across the zone, screaming and they kept it in.
When Suzy Berhow first told her husband, Arin Hanson (aka Egoraptor), about her idea for Table Flip, she compared it to a Victorian dinner show, where Suzy and Barry would bring in a different guest every game. Polaris greenlit it under the assumption that the show would be framed to actually take place in Victorian times and provided the resources needed to film it as such, so Suzy and co-host Barry Kramer decided to take the new spin on the concept and run with it.
At the end of If the Emperor Had a Text-to-Speech Device, Magnus can be heard murmuring "my god", which is either his show of reverence to Tzeentch or his voice actor catching his breath after almost a minute of repeating "rabble rabble rabble".
In Epic Rap Battles of History "Genghis Khan vs. Easter Bunny" there's a part where Lloyd (playing Khan) smashes an Easter egg with his sword. The behind-the-scenes reveals that they thought the idea wouldn't work well, so they only did one take before moving on. When Lloyd actually does hit the egg in one take, they decided to add it to the video.
A decent part of the plot of Ruby Quest occurs because Weaver took a few of /tg/'s silly and/or unexpected suggestions seriously. When somebody asked to put a severed hand up a pneumatic mail chute, up the hand went, and it was later used to reveal that Ruby used to be murderously violent. When /tg/ wanted to bash their way through puzzles, those puzzles were very quickly solved. When /tg/ directed Ruby to lick the Fleshy Prototube, they discovered it tasted bad. When the players voted to save Jay, three people escaped the Metal Glen instead of two.
The Angry Video Game Nerd's review of Kid Kool has a literal example of throwing something in that ends up thrown in. It ends with him throwing the cartridge blindly over his shoulder... and landing it perfectly in a garbage can by sheer luck. His look of amazement at the camera was genuine.
A few times in RWBY, the actors were told to add in their own lines from time to time. Notable lines being Yang's "Nailed it." after her landing in the forest and Ruby's comment about Blake liking to eat tuna a lot.