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Nightmare Fuel: Yogi Bear
The New Yogi Bear Show has the episode "Yogi the Cave Bear" where an earthquake opens temporarily a hole in the bears' cave, leading out into a Flintstones-like stone age version of Jellystone Park. All seems fine until we meet the bears' former selves, who are nothing like what we'd expect from a Flintstones episode. It doesn't help that the main characters could have been trapped forever in that world if they didn't come back home in time.