Machinima as a genre took off largely due to Red vs. Blue - and Red vs. Blue used a glitch that caused magnums to look like they were 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.
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.
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 without shouting "Zuul, muthafucka! Zuul!"
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.
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.
According to the commentary on Doctor 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.