In the newest Adventure Time spin-off Fionna and Cake, a popular theory involving Jake's wish to Prismo The Wishmaster was seemingly debunked by the last two episodes and a new interview from showrunner Adam Muto.
To quickly give context to those unfamiliar with the original Adventure Time cartoon series, the show's villain, The Lich, had the singular goal of killing all life. To accomplish this feat, he sought out a cosmic being named Prismo to grant him a wish: "The extinction of all life."
After tricking the show's protagonists, Finn and Jake, into helping him open a portal to Prismo's Time Room, The Lich got what he wanted from the Wishmaster and disappeared. Thankfully, the two followed him through the portal, allowing them an opportunity to undo The Lich's wish, with Jake succeeding.
To undo The Lich's extinction-level wish, Jake wished (with some helpful coaching from Prismo) that The Lich had instead wished for him and Finn to go back home to Ooo, both undoing his wish and safely sending them back.
In the original Adventure Time (re-emphasized in this spin-off), if a wish is of significant enough magnitude or change, instead of altering the wisher's original universe, Prismo transports the wisher to a "wish-altered" reality crafted around their wish.
Fans had assumed that The Lich's wish and the world created from it never happened, but Fionna and Cake proved otherwise, reigniting a brief but wild theory among fans.
Jake's Wish Theory
The theory goes that, instead of Prismo sending Finn and Jake back to their original universe after The Lich's wish was changed, he instead sent them back to a newly made, identical universe while their original was still killed by The Lich's wish.
After all, Finn's attempt to wish The Lich out of existence created its own universe, called Farmworld, that wasn't erased by Jake's wish, so the same principle applies to The Lich's wish.
However, this theory gained resurgence when in Adventure Time: Fionna and Cake, a dead universe inhabited by only one person, The Lich, was visited by the titular genderswapped duo.
In the episode "Jerry," the universe-hopping heroes run across a world that was seemingly killed by The Lich. However, fans quickly realized that this Lich (right) was the same Lich (left) who wished for the extinction of all life, wearing the skin of Billy as a meat suit (albeit about two decades more rotted).
Some fans suggested that this could be the original Adventure Time universe from seasons one through four killed by The Lich's wish, especially since it's suggested by BMO that the Finn and Jake of that world disappeared shortly before everyone turned into skeletons.
It's as if the original Finn and Jake ran through a portal after The Lich one day, never to return.
Debunking a Dark Theory
However, even before the two-episode finale, there were holes in this theory, namely a specific note left behind by Finn.
Canonically, this note could not have been written by Finn before he disappeared chasing The Lich because he only remembered the meaning of the acronym BRB and wrote this same note, seen above, in the tenth season episode, "Blenanas."
Additionally, in a recent interview with The Beat, Muto reminded fans that "the logic [they'd] already set up" with Prismo's wishes them having "some sort of monkey's paw drawback:"
"Because the logic we’d already set up was that most of these new multiverse dimensions are created by a wish and there was usually some sort of monkey’s paw drawback to them, that the person was unaware of and was unintended."
While it was only suggested in the episode "Jerry" that The Lich was depressed, the start of "Casper & Nova" confirmed it in his plea to GOLB, the embodiment of chaos.
It's clear that the "monkey's paw" aspect of The Lich's wish was the realization of his goal giving him no satisfaction. Combined with previous evidence, it's far more likely that Prismo did indeed make a new (dead) universe for The Lich's wish and sent Finn and Jake back to their original reality.
The first season of Adventure Time: Fionna and Cake is currently streaming on HBO Max.