I often find some idea repeated a lot in movies, comics, etc. And I am always curious what the first example of such a trope is, sometimes the first example in a given medium, and sometimes the first "modern" example.

If anybody has some tropes they are curious about, name it, give us some examples, and we'll see if we can come up with more examples of the same, the earlier the better.




Here's an example of what I mean:
The idea that the superhero's teen sidekick grows up to be evil.

The examples below all spoil the surprise villain:





"Batman Beyond: Return of the Joker": Tim Drake grows up to be the Joker -- 2000
"Scooby Doo": In the film, Scrappy Doo grows up to be the surprise villain -- 2002
"The Incredibles": Young Buddy Pine fails in his attempt to be Incredi-Boy, sidekick to Mr. Incredible, but grows up to be a formidable nemesis, Syndrome -- 2004
"Winter Soldier": Bucky Barnes turns out to be alive and the villain the Winter Soldier -- 2004
"Under the Red Hood": Jason Todd turns out to be alive and the villain the Red Hood--2005

How common is this idea? What are the earliest examples? What tropes are you curious about?