Mixed with No Ending in "Evicted!". After a whole episode of dealing with Marceline taking over Finn and Jake's house, she gives it back... and they return to find a giant worm that blasts them with a ray of some sort and asks them to hug it. It got a Mind Screwdriver of sorts in "King Worm"... three seasons later! (And it also had an ambiguous ending, since it mirrored the final scene from "Evicted!".)
None of these even come CLOSE to the spectacularly weird ending of "The Other Tarts." The psychotic Tart Toter bursts into the castle, brandishing a chicken and a squirrel in place of tarts, foaming at the mouth. He says: "This cosmic dance bursting with decadence and withheld permissions twists all our arms collectively, but... If sweetness can win, (and it can!), then I'll still be here tomorrow to high-five you, yesterday, my friends. Peace." As he says this, the camera zooms in on his deranged, foaming face, and fades to the Tart Toter's delusion of drifting through space, surrounded by sweets, and LSP is in the space background, taking a donut. All of a sudden, it cuts back to the castle, where Finn cringes and says "Geesh...", and the episode ends.
The ending of the season 4 finale "The Lich". In an attempt to stop The Lich from escaping through a portal conjured by the Enchiridion, Finn and Jake go through the portal and... we see what appears to be an alternate universe where Finn's clothes and face are different, he has a crudely-made prosthetic right arm, he plays the flute, Jake is a regular dog, and they live on a farm with Finn's mother. The episode ends when she asks Finn to come inside for something urgent.
It was a cliffhanger to the events of season 5's two-parter premiere, "Finn the Human" and "Jake the Dog".
Note the fact that before that sequence began, Banana Man was shown passed out in the pool. Maybe the sequence after that was him dreaming.
"Hug Wolf" ends with Finn managing to defeat the Alpha Hug Wolf who turns back into a female humanoid. Jake asks her for a hug to which she suddenly turns into the Tree monster from earlier, cue Jake screaming, cut to black.
The end of "Dungeon Train" where Finn shows Jake the future orb showing them washing dishes hinting that Finn won't be on the train forever. The episode then ends abruptly.
"Box Prince". Finn gazes at the remains of the "kingdom" with the cats playing around and wonders if it even was a kingdom. The cats then dog-pile on Finn and purr. The end.
"Red Starved". Marceline sucks the red (or pink) out of Princess Bubblegum, and PB ends up as a deflated, grey sack. Then PB asks Finn for the "Spoon of Prosperity" that they were using at the beginning of the episode. PB puts the spoon on her nose which somehow turns her back to normal. They all leave the cave inside a giant sand worm.
"Escape From the Citadel" ends with one of the most bizarre twists imaginable, with the show's Big BadThe Lich being reduced to a baby who is apparently going to be adopted by Mr. Pig and Tree Trunks, and is possibly the sole reason they will remain married.