Episode 001 of Adventure time:
Finn and Jake travel up a mountain on an adventure and go through a number of trials so that they can have a picnic and read a book on how to kiss a princess.
Episode 266 of Adventure Time:
Finn's duplicate, which was formed from a cursed grass sword that had become his arm after it was ripped off by his father, fused with a sword made out of finn himself when he was paradox cloned by the god of the multiverse so that jake could make said god come back to life by turning into him into his sleep, takes Finn into some ruins. There he locks Finn up with nothing but trash and dirt to sustain himself, with the intent of taking over Finn's life by transforming into him. Finn escapes after realising Princess Bubblegum had put voice functions into his robot arm that replaced the grass arm which was now his duplicate. After this, a desperate Finn assassinates his grass altself by accidentally using the "Fatality" function which was programmed into the robot arm. He goes home truly horrified about what he had done, but little does he know, Princess Bubblegum's presumably exiled Uncle Gumbald who could be a thousand years old and looks like a B movie villain shows up and picks up Fern's pieces.
....First episode of Adventure Time:
Princess Bubblegum straight up brings her ex-boyfriend back from the dead, then creates a chain of events that result in her own death 2 seasons later.