9. This Side of Paradise a.k.a. The One Where Spock Totally Gets Some (Episode 24)
The Plot: The Enterprise is sent to Omicron Ceti III, where a colony of humans have been destroyed by deadly Berthold rays. However, they find the colony not only alive, but all in beyond perfect health to the point where childhood injuries have repaired themselves. The head of the colony says it's because of their "hippie vegan commune lifestyle, man," and "the planet's plant life, is just that mondo groovy." Kirk hates hippies, so he makes them all stop and totally harshes their mellow.
The Screw Up: This is something that many shows, books and movies are guilty of: the discovery of something that would change the entire universe the characters are in, but is only used once and never referred to ever again (example: Harry Potter's Time-Turner.) This plant can heal disease and regrow missing organs and tissue. The implications for such a plant would change the entire Star Trek universe, as it is a cure for death and illness! To quote Judge Snyyder at the end of The Simpsons' episode where it was revealed that Seymour Skinner had been an impostor all along, "I further decree that everything will be just like it was before all this happened! And, no one will ever mention it again under penalty of torture!"
Cool Trivia: This episode was parodied in a segment on the great Mystery Science Theater 3000 episode Swamp Diamonds.