Based on the search results, there are different interpretations of the question "what happened to baby Finn?" Here are the possible answers:
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Adventure Time Baby Finn: According to the Adventure Time Wiki, Baby Finn got a hold of a rattle and hit Baby Cinnamon Bun in the head, causing him to cry. After Simon reassured Fionna that they will find a way to fix the situation, the episode ends.
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Adventure Time Baby World Finn: In a Reddit thread, a user asks what happened to Baby World Finn, who was last seen in the Pepbut tank. Another user replies that Bubblegum and Marcy supposedly plummeted him in that universe, leaving him behind.
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Adventure Time Fionna & Cake Baby Finn: In a YouTube video, the narrator describes a reality where everything is or is made for babies, and nobody ages. In this reality, there is a Lich who is actively isolated from the other children mainly by a gumball Guardian. The narrator does not explain how Baby Finn came to be in this reality.
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Real-life Baby Finn: According to Finn the Hero, Finn Benton Pope was a baby boy who died from a brain aneurysm at 11 months old. His parents created a website and a foundation in his memory to raise awareness about brain aneurysms and support research.
Therefore, the answer to "what happened to baby Finn?" depends on which Finn the question refers to.
