This is based on opinion. Please don't list it on a work's trope example list.
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 The 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.
The infamous poster for the Live-Action Adaptation with the tagline, "great things come in bears!" The fact that it uses really close-up stock images of Yogi and Boo-Boo right on top of each other is a little unnerving.