***1). The Adventures of Superman:***  
During the golden age, Superman's powers came from Krypton's high gravity. The *Superman* radio show, which had no reruns, played loose with this lore to give the voice actor breaks. *Kryptonite* was invented specifically to let Bud Collyer take a vacation.  
This would be expanded upon in the silver age. Superman's powers would then (at least partially) come from Earth's yellow sun. With Kryptonite emitting a radiation poisonous to his sun-absorbing tissue.

***2). Adventure Time:***  
During early production of season 1, "the land of Ooo" was a typical fantasy world. In S1E8, Finn and Jake find modern "businessmen" frozen in ice. Creator Pendleton Ward decided this must mean Ooo is actually a post-apocalypse Earth.   
This lore is explored in later seasons. With Ooo existing 1,000 years after the catastrophic "Great Mushroom War" obliterated human society and returned magic to the world.

***3). The Venture Bros:***  
In S1E10 of The Venture Bros, Dr. Venture hosts a yard sale for his super-science inventions. To explain why villains were allowed there, the writers came up with "*the Guild of Calamitous Intent*." Which regulates the rules and laws of protagonist-antagonist conflict.  
It was initially a one-off joke, but the Guild later became a central plot point for the series. With major arcs exploring what happens if a disgruntled villain decides to break the Guild's rules.

[Loved trope]: Major worldbuilding comes from (an initially minor) production decision