Adored by the Network: The show is this. Heck, one might even say it saved the network (since, prior to this show, the channel was going downhill, thanks to the classic cartoons being moved to Boomerang, the Cartoon Cartoons ending/getting cancelled, the push for more live-action shows [which, for a channel dedicated to animated shows, was a very stupid idea], and behind-the-scenes politics and double-dealings)! Very much adored along with Regular Show and, to a lesser extent, The Amazing World of Gumball. Name any other Cartoon Network show, before or since, that's gotten as much hype, as much merchandising, or as much publicity as this one.
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.
Sort of. Liz Prince wrote a 7-page story called "Fruit Salad Days" for Marceline and the Scream Queens Issue 3, in which Lemongrab was the frontman of a punk rock band called "Unacceptable." Liz Prince's idea of Lemongrab having a punk band named Unacceptable was actually re-used for an official Adventure Time game and comics the following year.
His 13th birthday episode first aired, coincidentally, on Pi Day (March 14, 2011) which first Fanon and later Word of God made his birthday.
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.
Fred Siebert has talked about how Cartoon Network tried (and failed) to get rid of Bubblegum and Marceline because they didn't want the show to appeal to women.
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".
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.
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.
In "Davey", to make his disguise more authentic, Finn changes his vocal chords to a more adult tone of Davey Johnson.
Cosmic Owl has a more masculine voice than his original voice.
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 the relationship between Lemongrab and his clone. Princess Bubblegum and Marceline used to get a lot of this to until Olivia Olson finally just flat out confirmed that they're ex-girlfriends.
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".
Justin Roiland voices Lemongrab, Lemongrab's clone, and all of their children.
Teasing Creator: A lot, if not most, of the staff count. Exhibit A, Adam Muto:
The staff loves to remind the fans that Fionna and Cake and related characters will never be part of the main AT universe.
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.
In "Goliad", the titular character (voiced by writer Graham Linehan’s daughter) badly flubs the line "I'm Don Juan Cherry Tempo!" but the producers clearly thought it was so adorable and funny that they left it in.
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.
Seeing as some fans ship the two Lemongrabs together, "Another Five Short Graybles" has the original Lemongrab eat the other, as well as revealing that they view each other as brothers rather than lovers.
Voices in One Room: The cast records their dialogue together in one room, which is rare for an animated show.
Western Zodiac: Fitting enough that Finn's element is water. His birthday, March 14th, exists in the Pisces period during the year, and Pisces' Zodiac element is water.
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 episode "Betty" was supposed to be a double episode, but there was already a double episode in production and Huntress Wizard appeared in a 30 second scene but it was cut for unknown reasons, possibly time reasons.
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.
Lead character designer Andy Ristaino revealed that the Gladiator Ghosts are indeed homosexual and in love with each other after "Morituri Te Salutamus" aired.
Kate Leth draws the Lady Lemongrabs as a couple. In the Fionna and Cake comic book, their relationship is only suggested through (admittedly obvious) subtext. Outside of the issue, though, Leth repeatedly draws the Lady Lemongrabs in obvious romantic scenarios.
Olivia Olson, the voice of Marceline, revealed that someone on the show's crew told her that PB and Marcy used to date, but that they can't outright say it on the show because of Moral Guardians.
Word of God: Princess Bubblegum is actually made of bubblegum.
Princess Bubblegum's composition is actually a plot point in "Mortal Recoil".
BMO is "sexless", being a robot, and also genderless. S/He has been called masculine or feminine pronouns by different characters. For what it's worth, BMO is voiced by a woman, and the episode "BMO Noire" uses a feminine spelling; however, that same episode has BMO imagine having a relationship with a female (chicken), while in another s/he pretends to be a "real boy."
After years of hints in-show Olivia Olson finally confirmed that PB and Marceline are ex-girlfriends (as if it wasn't blatant enough already).
Working Title: Many of the episodes have had original titles that were changed. Some examples:
"The Royal Promise" → "Slumber Party Slaughter" → "Slumber Party Panic"
"Meet the Ice King" → "Prisoners of Love"
"Ice King and Son" → "What Is Life?"
"Return to Nightosphere" → "Marceline's Dad" → "Nightosphere" → "It Came from the Nightosphere"
Tree Trunks was based on as well as written expressly for Livingston, who is an old friend of Pendleton 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.