One fan made a drawing of a tiny cat assassin named Me-Mow. The creators liked the idea, and thus, the episode "Jake vs. Me-Mow" was born.
Bad Export for You: The Cartoon Network Australia version of Adventure Time has a truckload of stuff cut out due to censorship, making a lot of episodes look and sound choppy and incomprehensible...
Banned In China: ...and episodes that aren't butchered are merely pulled from rotation (such as "Fionna and Cake" and "What Was Missing" due to many scenes deemed too sexually suggestive). Thankfully, when the series appeared on free-to-air it was screened unedited (though a few episodes were out of order).
In "Storytelling" Finn says "aminals" instead of "animals", although given the Adventure Time's use of odd slang and particularly Finn's fondness for blurting out his own nonsensical versions of words and combinations of words this mispronunciation may be intentional.
In "The Limit" Jake says he can't blink... and then blinks mere seconds later.
They forgot to put the snail in "Trouble in Lumpy Space" and "Blood under the Skin".
In "Business Time", they misspell Finn's name as "Fin".
In "You Made Me!", Lemongrab's shoes randomly disappear when he takes out the Sound Sword for the first time.
Ghosts: Farewell, Pat. Good luck in New York. Come back soon.
Executive Meddling: According to Word of God, originally the ending of "Tree Trunks" had Tree Trunks just explode after eating the crystal apple. Cartoon Network made them add the scene where she is inside the crystal. It's hard to say if said meddling affected "Crystals Have Power", in which she is shown to be completely alive and healthy, and Finn and Jake manage to snap her out of being villainous.
Also completely altered the ending of "His Hero", which originally would have ended with Billy's throne being revealed as a Sealed Evil in a Can once Billy gets up, leading to a You Shall Not Pass moment. The new ending does get some crap past the radar though.
Fan Nickname: The muscular robot-armed Finn from "Mortal Folly" has been named Guts Finn by users on /co/.
Fans often call Alternate Universe Finn "Huckleberry Finn."
Lady Rainicorn and Jake's offspring are often called "corndogs".
Flip Flop of God: At one point, the creator (Pen Ward) said that Tree Trunks was definitely dead. But then "Crystals Have Power" came along....
God Never Said That: Andy Ristaino (character designer) never said that "What Was Missing" was a musical. He said that the episode would kind of resemble one because it has a couple of songs in it.
This was the reason for the whole "BubblegumXMarceline Mathematical" fiasco. Frederator took down the video because it made it seem like the show was openly asking for plot input for further PBxMarcy development and Adam Muto, the storyboard artist and writer of the episode, didn't like the video framing that interpretation as only being romantic.
Hey, It's That Voice!: Much like The Simpsons, this show has a lot of celebrities brought on to play characters (some are popular voice actors, while some aren't). Unlike The Simpsons, the celebrities never play themselves, nor is their dialogue lampshading their career and celebrity status. It's truly amazing when you look in the credits and find out who's who.
Meaningful Release Date: "Mystery Train" (the episode featuring Finn's 13th birthday) aired on March 14th (3/14 or "Pi Day"), which sorta means that Finn's birthday is on Pi Day. Mathematical!
Network to the Rescue: Nickelodeon planned to develop the short into a series, but pretty much ignored it. When the rights of the short returned to Pendleton Ward in 2007, the idea was pitched to Cartoon Network and greenlit a year later.
Played straight with Zack Shada. Zack hit puberty and he couldn't get his voice to sound like Finn's anymore, so when the show went from being a Nickelodeon pilot to a Cartoon Network series, the voice director got Jeremy Shada (Zack's younger brother). Now averted with Jeremy, as rather than get rid of him because of his voice changing, the writers incorporated Jeremy's vocal change to Finn's.
Princess Bubblegum is played by Paige Moss in the pilot and Hynden Walch in the series.
In the pilot, Lady Rainicorn was voiced by Dee Bradley Baker and spoke nonsense. In the show, her voice actress is Niki Yang (the same Niki Yang who plays BMO) and the nonsense was replaced with fluent Korean.
Ghost Princess is played by Pen in "Loyalty to the King" and Maria Bamford in "Ghost Princess".
In the video game, Marceline is played by Jessie Seely instead of Olivia Olson. Granted, the difference is barely noticeable.
Playing With Character Type: At first, the Ice King seems no different than Tom Kenny's usual characters, until the Christmas Episode "Holly Jolly Secrets" where it's revealed that The Ice King was once a man named Simon Petrikov and that he became the Ice King after wearing a crown that drove him to insanity and may have had something to do with the disappearance of the woman he loved.
Real-Life Relative: Olivia Olson is the daughter of Martin Olson, and they voice Marceline and her dad, Hudson Abadeer (respectively).
The same answer is usually given regarding the alleged relationship of Princess Bubblegum and Marceline, and the relationship between Lemongrab and his clone.
Spoiler: Though the spoilers don't actually happen "in-show". A lot of the season one storyboards could be viewed online before their episodes even aired. But hey, it would be fun to watch the episodes to see if anything got changed.
Pen Ward blatantly mentioned at Comic-con that Flame Princess would be Finn's new love interest before "Incendium" aired. However, he didn't spoil her destructive personality and only called her "intense" (a word that a member of the audience laughed at).
TV Guide spoiled the fact that the short stories in "Five Short Graybles" are centered around the five senses (sight, touch, taste, smell, and hearing). It wasn't supposed to be revealed until the end of the episode. It really makes "Huh? You still haven't figured it out?" less funny.
While it's not clear if any of her characters talked to each other, stand-up comedian Maria Bamford has voiced twenty-six voices for the show, even doing male voices such as Manfried in "Slumber Party Panic" and the Butterfly Bandit in "The Other Tarts".
Teasing Creator: A lot, if not most, of the staff count. Exhibit A, Adam Muto:
Throw It In: An unusual example. When Adam Muto originally pitched "What Was Missing", he flubbed the original line that read "I'll get your toy back, kid!", and he said "I'll get your kid back, toy!". They thought it was funny that way, so they kept the line flubbed.
A lot of the fandom sees the show's writers as this. For example "Fionna and Cake" basically compares the Ice King to fanfic writers. Many fans see "All the Little People" as a Take That and parody of shippers in general. And then there's the end of "Mystery Dungeon" where Ice King didn't get to make the Fionna and Cake world real, which is what many fans wanted.
The original storyboard for "The Duke" has Jake imagining how evil the Duke can be. The Dramatization includes a particularly impressive Hurricane of Nut-Puns by the Duke, which includes "CASHEW LATER, COPPERS!" and "CHESTNUT YOUR DAY, JAKE!". However, this was scrapped for time.
The Princess Plant was originally supposed to be called the Halo Lily.
As mentioned before, the fact the series was originally to be on Nickelodeon.
The original ending of "Sons of Mars" would have shown Death transporting Lincoln back to Ford's Theater before fading to black with the sound of the gunshot. The final version has a gunshot, but no Ford's Theater, implying but not making explicit the events shown in the storyboard.
There were also several changes to "The Lich". For one, Prismo is called "The Almighty Grob", but more importantly the Lich would've been possessing Billy instead of having killed and replaced him, and Booko would have revealed that Finn's human dad was a hero who was wrongfully imprisoned in an impossible-to-escape dimension—with the implication that Billy put him there—and is still fighting the other inmates. This ties into several examples of The Artifact. In the actual series, "Grob" refers to a powerful (but not all-powerful) magical being from Mars (full name Grob Gob Glob Grod). This storyboard also reveals that Finn never realized that he was adopted (with a Lampshade Hanging on the ridiculous levels of Idiot Hero that would come along with this notion), which is obviously not the case in the series. Sadly this meant one of the series biggest heartwarming moments ended up being cut as well.
According to Jesse Moynihan, Lemongrab was not originally a socially awkward, dysfunctional, brain-damaged, unadjusted Jerkass Woobie in the original outline. He was originally conceived by Pen to be a one-shot, one-dimensional Jerkass "like Xergiok." However, Jesse stepped in, and decided to give Lemongrab the puzzling amount of depth he has, by making him "weird, isolated, angry, off-putting, with an inability to read social cues." Part of Lemongrab's appeal appears to be his many woobie-ish and sympathetic traits. If Jesse hadn't fleshed out Lemongrab's character, LG wouldn't have the zealous fanbase he has, and he probably wouldn't have been considered to make a return in the series after his exit from "Too Young".
The uproar that happened over the Ship Teasing in "What Was Missing" caused "Mathematical!" to stop producing recaps.
Nat and Pen used to make themselves known on /co/, but they stopped due to... obvious reasons. Nat later came back for the show's second anniversary. She also came back the Sunday night before the season 4 finale.
Tree Trunks is both based on and voiced by an old friend of Pen Ward's family.
Not so much a person, but a urinal where Penn works was the inspiration for the Cosmic Owl. The light reflects off the very shiny metal urinal and resembles the Cosmic Owl. This strange little tidbit of triva came from a interview with Nintendo Power.