Jake in "Prisoners of Love:" "Yeah I know what 'Ice King' means! A BIG NERD!" Later episodes reveale the Ice King used to be a well-read archaeologist who published multiple books on the occult and arcane before the crown drove him insane. He really was a big nerd.
Many people complained that "Incendium" felt short and should've been a two-parter. Turns out that the season 4 premiere continues directly from where "Incendium" left off, so the two episodes may as well be considered parts of the same episode!
This image◊ becomes a lot more relevant after watching "Lady and Peebles".
Justin Roiland (Lemongrab's voice actor) apparently predicted the plot of "All Your Fault" (and parts of "You Made Me") two years before the episode aired, in a strange freestyle rap from his podcast. It's uncanny to say the least.
In the Japanese dub, Finn is voiced by Romi Park, who is Korean-Japanese and speaks both languages as well. And he can't understand Lady Rainicorn. Even more hilarious (and ironic) she also voiced by another Korean-Japanese actress, Bong Young-hi.
Lemongrab's very first line in the show was "This castle is in unacceptable condition!" In the episodes "All Your Fault" and "Too Old," his castle is in very unacceptable condition.
Some fan art reinterprets Me-Mow as either having a more humanoid figure and/or being much taller. Several seasons later, when she does finally appear again, she's taller than Finn and has a feminine shape clearly more like a young woman rather than a Funny Animal.
Cloris Leachman voices an elderly undead creature whose last words while in her skeletal form were "I Warned You, you butt" right before she gets decapitated by the protagonist. Leachman would later on play another elderly undead creature in the form of a zombified Crazy Cat Lady whose scene involves toothlessly nibbling on a protagonist's butt also right before she gets decapitated by a protagonist in Scouts Guide to the Zombie Apocalypse.
In "The Cooler", Princess Bubblegum infiltrates the Flame Kingdom so she can disable their weapons of mass destruction before the Always Chaotic Evil country can use them against hers only to get caught by Flame Princess, nearly starting the very war she was trying to avoid. In Attack on TitanMAJOR SPOILERS it's revealed double-agents infiltrated the walled city so they could prevent the "evil" humans inside (defectors fleeing persecution and potential persons of mass destruction) from using their hidden weapons of mass destruction. The attack winds up exposing the weapons to the civilians who didn't know about them, the enemy country, or anyone existing outside their walls, panicking the only person in the whole city who knew how to use the weapons into passing his power to another, and giving that person many, many, many reasons to attack the agents' country. Additionally both PB and the double-agents should know their "enemy" isn't planning on attacking anyone but they think they're clever enough to risk provoking them anyway.
Finn's father Martin was jailed for years as a cosmic criminal, for about the same amount of time Finn's mother Minerva kept nearly all the remaining humans on Founders Island — essentially as a jailer.
In "Preboot," Dr. Gross says that the next logical step forward for human body modification is "scissor-hands and telescopic spider-legs." Fast-forward to 2018, and it seems that Mac Gargan took her advice to heart.
In "Her Parents", Jake nervously greets Rainicorn's parents with a gratuitous "Annyeonghaseyo" (Korean for "Hi" or "Hello"). Years later, thanks to the surge of popularity towards South Korean culture due to K-Pop, "annyeonghaseyo" has become something of a meme among K-Pop fans.
In "It Came From the Nightosphere," Marceline sings about how her father ate her fries and how she reacted. One year later, a completely different Cartoon Network show would base an entire episode around a character overreacting to another character eating their fries.
It turns out that the crew must really like high-fives since this clip from "Five Short Graybles"
