9. Tribbles Exist Thanks To A Gentleman's Agreement
Non-intelligent balls of fluff, Tribbles are an alien species that are born pregnant, and when given access to abundant food can reproduce at an alarming rate. Introduced in the TOS episode The Trouble with Tribbles they are a beloved staple of the Star Trek Universe. They have appeared in all different Star Trek series from a sequel episode in the TOS animated series, a crossover episode of Deep Space 9, an episode of Star Trek: Short Treks, and even showing up in the film Star Trek Into Darkness in a scene where Bones tests a serum from Khan's blood on a dead Tribble.
But these beloved fuzzballs might not have existed at all, if not for a gentleman's agreement. When writer, David Gerrold, turned in his script it was noted that Tribbles bore a lot in common with another alien creature, Flat Cats, from the Heinlein 's novel The Rolling Stones.
So similar was the idea of the aliens (small, fuzzy creatures with a soothing purr that reproduce when fed to the point of overwhelming a spaceship), that it was suggested they should purchase the rights to the story outright so as not to risk claims of plagiarism. Instead, Gerrold called Heinlein, and the author agreed to waive any claim to the aliens, allowing Tribbles to be born into the Star Trek universe.
In his autobiography Heinlein spoke of frustration about the whole ordeal. In retrospect he would not have made the agreement at all, saying " If the matter had simply been dropped after that one episode had filmed, I would have chalked it up wryly to experience. But the "nice kid" did not drop it; "tribbles" (i.e., my "flat cats") have been exploited endlessly."