Throw It In/Western Animation

"Jeepers: Hehehehehe... You want to see something strange and mystical?
 * On The Simpsons, the often-quoted sung line: "I am so smart! I am so smart! S-M-R-T! I mean, S-M-A-R-T!" This was an actual mistake by voice actor Dan Castellaneta, but it was just so "Homer" they left it in.
 * Homer's famous "D'oh!" was improvised by Castellaneta. The script only said "annoyed grunt". (And still does.)
 * As was Homer's song "Dancing Away My Hunger Pangs" on the season 12 episode "Hungry, Hungry Homer" (where Homer goes on a hunger strike to save the town's baseball team).
 * Recurring guest star Al Brooks is known for doing this. Noticeable when he plays Hank Scorpio in You Move Only Twice - most of his fast, random declarations are improvised. The entire Hammock district routine is improvised - hence why Homer only replies with a quick "Yes" because Dan Castellaneta had trouble keeping up with Brooks. Only at the end does he catch up and reply "Oh in the hammock district".
 * In one episode, when Snake attempts to murder Homer with a trap utilizing piano wire in order to get his car back, the trap missed Homer, but manages to amputate Mr. Van Houten's arm while he was complaining about his sandwich. Originally, the piano wire was simply supposed to cut the sandwich to what he wanted it to be like, but the animators decided to make it a bit more shocking by having it cut off his arm instead.
 * Megatron's famous "yeeeeeeeessss..." from Beast Wars wasn't originally scripted, but was improvised by voice actor David Kaye, who thought it fit the character. It did, and quickly became his Catch Phrase.
 * David also coughed while recording one of Megatron's power-hungry laughters, but they decided to leave it in, with the animators adding a surprised reaction shot of Inferno to complete the effect.
 * Futurama's famous Hypno-Toad sound effect was meant to just be a placeholder until a proper sound effect could be edited in. However, the placeholder noise sounded "so wrong" that the creators decided to keep it.
 * An in series example is where Calculon is showing the clip to a new episode of his show where he does a Big No. He informs the talk show host he was actually supposed to say "yes" but kept it in.
 * Invader Zim
 * The first episode contains the line 'Invader's blood marches through my veins - like giant, radioactive rubber pants!' The line was supposed to say 'ants'. But it's the kind of show where "pants" actually fits better.
 * The Hamstergeddon episode involved Ultra Peepi swaggering down the street, not by intention. It came back from animation that way, and Jhonen loved it and had the composer make funk music to go under it. "I want to hear Barry White saying 'Ultra Peepi.'" Check it out here.
 * According to Bruce Timm, Terry's revelation as  in Batman Beyond and the Justice League Unlimited episode "Epilogue" stemmed from the earlier realization that, , Terry's black hair was genetically implausible. Apparently, We Will Not Use Hair Dye In The Future.
 * Shego from Kim Possible was originally created as a generic, boring Dragon to Dr. Drakken, but after the creators heard Nicole Sullivan's performance as Shego they gave her a bigger role, making her sarcastic, more competent and far savvier than Drakken.
 * Freakazoid's voice actor was supposed to refuse a plot hook with a "No," and the sequence ended up twenty seconds longer because of what he did say.

Freakazoid: NO! Get out of here with that watch! Lay off the poor beavers, will ya?! SHEEEEESH! You're a creep! Go away! We were having a good time until you showed up, Jeepers! Urrrg! Go have some coffee with cream or something! Because I'll tell you something... This is a happy place!"

"(Porky sends his dog after a duck, but instead of letting himself get caught, the duck picks up the dog and dumps him on the shore of the pond)
 * According to the audio commentaries, this happened a lot. Even the casting of writer Paul Rugg as Freakazoid was an accident; he was just supposed to be the placeholder, but ended up being so funny (in part because of his wild improv), that he was cast permanently. Apparently, several large sections of the first episode were entirely made up on the spot by Rugg.
 * Looney Tunes. Bob Clampett's 1943 short An Itch in Time has a scene of a dog, having been irritated by a flea, scooting wildly around a carpet on its rear end. As an "inside" joke, Clampett and his animators added an off-color gag of the dog pausing to tell the audience, "Hey, I'd better cut this out, I may get to like it!", fully expecting that either the studio or the Hays Office would have it removed. Astonishingly, they didn't.
 * This was apparently par for the course -- they'd include really extreme stuff to distract the Hays office so the stuff they really wanted in would scoot by, only for the Hays office to ignore the decoys and go for the real meat anyway.
 * "SMOOOOOOOOOOOOOOOOGGGGGGGGGG!"
 * Happens in-universe in Porkys Duck Hunt, the cartoon which spawned Daffy Duck. (In a real-life example, the duck was never intended to be anything more than a one-off gag, and he was retroactively named "Daffy" when he proved to be extremely popular).

Porky: Hey! Th-th-that wasn't in the script!

Daffy: Don't let it worry ya, skipper, I'm just a crazy darn fool duck. (bounces away across the pond, laughing maniacally)"

"Announcer: Dead African child indee-hee-hee-hee-heed... (uncontainable laughter), I'm sorry. It's just so f-bleep-ing awful."
 * The Woody Woodpecker short Wet Blanket Policy originally didn't have "The Woody Woodpecker Song" in it, but it was added in at the last minute by Walter Lantz, when he discovered how much of a surprise hit the single had become.
 * The episode "Shrabster" of Sealab 2021 involves an hammy, over the top announcer declaring "Indeed!" along with a linking remark with each scene change. The hammyness ultimately becomes too much for the announcer when leaving a clip involving a starving 14 year old African boy.


 * MEANWHILE!!
 * Avatar: The Last Airbender:
 * Sokka was originally intended as a more sullen character. However, his voice actor was a Jim Carrey-inspired comedian, so Sokka gradually turned into a much more over-the-top comic relief. The creators noted that because of this change, Toph ended up filling Sokka's original role.
 * Also, the famous Foamy Mouth Guy was just going to faint in the background. Then an animator got really creative...
 * Haru's SEXYFINE 'stache came about because a bored animator snuck it in.
 * Sequel Series The Legend of Korra has a scene in "The Spirit of Competition" where Mako is angrily walking through a door and completely ignores a fan trying to say high ("Hey Makooooo-oh"). This originated when someone drew a random extra standing next to Mako in the animatic and the episodes director thought he looked disappointed.
 * In the Code Lyoko episode "Tidal Wave", Odd's final line, "I'm so hungry!", was ad-libbed by his dub actor.
 * In the Daniel and the Lions' Den segment of the first Veggie Tales, a render glitch made a sudden flash of light in the background during a scene. The animators weren't able to fix it, so they just added a thunder sound effect and made an instant storm, that actually darkens the mood in the right way.
 * In My Little Pony Friendship Is Magic, in the first episode, a pony in the background was given crossed eyes as a joke by an animator, and wasn't intended to be visible. Last minute changes moved a bunch of other characters out of the way, making this pony visible. Fans noticed, and were amused, leading to the pony being nicknamed "Derpy Hooves." The joke became so famous that the show's creator, Lauren Faust, said "she can be Derpy if everyone likes."
 * They also gave her the cross-eyed look in most other episodes she appeared in during post-production in season 1. In season 2, her appearances are now fully scripted.
 * A rather unhumourous example happened in The Ren and Stimpy Show Adult Party Cartoon spinoff that ultimately led to cancellation of the series. The episode "Onward and Upward" contained a graphic homosexual sex scene between the eponymous duo in the original run, added as a joke by an animator. Creator John Kricfalusi explained that there was never supposed to be any actual sex in it, that Ren and Stimpy were supposed to be playing baseball under the sheets (as a part of a vague "pitcher and catcher" metaphor earlier in the episode). He claimed he wanted to cut it straight away, but he felt sorry for the animator who thought it was funny. The scene was left in and caused a minor uproar at an animation newsgroup, where a group of people thought Kricfalusi was trying to sabotage his own show - the executives at Spike TV saw this conversation, thought that's the way everyone felt and cancelled the project. Go figure.
 * When building the models for the TV series TUGS, it was found that Zorran's eye mechanism caused the front of his hat to tip up. They left it in because they thought it added to the expressiveness of the character.
 * In the episode Munitions, a piece of debris from one of the explosions hits Warrior, bouncing off of him. A piece of dialogue was later added with him exclaiming "That bit hit me!"
 * According to Ashley Eckstein, she was asked to use an Icelandic accent for the voice of Ahsoka Tano in Star Wars the Clone Wars, but when she came in with a practiced Icelandic accent, it apparently wasn't what they expected. The voice she ended up getting told to do in the show is based on the tones in her real voice during the audition when she got frustrated that she couldn't figure out what accent they wanted.
 * In The Brak Show, Dad was originally intended to be around the same height of the rest of the main cast. A mistake in scale was made while compositing a scene, and the new, comically small version of the character was deemed funny enough that they stuck with that design.
 * Home Movies is so improvised, you can hear the actors stuttering and trying to come up with a new line.