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/.
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.
Natasha: because I wanted to see an Arthur kade article on the wiki
Meaningful Release Date: Wellll... accidentally. "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 didn't give the show its due diligence. When the rights of the short returned to Pendleton Ward in 2007, the idea was pitched to Cartoon Network and greenlit a year later.
The Other Darrin: Finn is played by Zack Shada in the pilot, but in the series, he's played by his younger brother Jeremy Shada because Zack's voice got too low. Kind of ironic- Pen stated that Finn will now age with his voice actor, so it doesn't really matter how low his voice gets!
Princess Bubblegum is played by Paige Moss in the pilot and Hynden Walch in the series.
Ice King is played by John Kassir in the pilot and Tom Kenny in the series.
Ghost Princess is played by Pen in "Loyalty to the King" and Maria Bamford in "Ghost Princess".
Real Life Relative: Olivia Olson is the daughter of Martin Olson, and they voice Marceline and her dad, the Lord of Evil (respectively).
Rerun: Occasionally during syndication, two weeks pass before a new episode airs. For example a week after "Hug Wolf" aired and a week before "Princess Monster Wife" aired, "What Was Missing" was rerun.
The same answer is usually given regarding the relationship of Princess Bubblegum and Marceline.
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.
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.
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.