These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Accidental Nightmare Fuel: Ed's mouth flaps sometimes cross into this territory, due to the relative crudeness of the stop-motion animation.
Bizarro Episode: The episode "Mako" revolves around an animated alien who comes to earth and takes over the bodies of whoever it goes inside. Its victims have no memory of what happens when the alien takes them over.
Ear Worm: The theme song is the reason most people remember this show.
Epileptic Trees: It's possible to interpret Mister Ed as the deranged hallucinations of a man who owned a completely normal horse, but in-universe evidence that Ed can talk is much stronger.
Fetish Fuel Station Attendant: Connie Hines as Wilbur's wife Carol. One episode had her doing alterations on a dress and she had the skirt hiked up to her thighs.