Superman, 1940

Conventional Supermanic History tells us that Kryptonite, Superman's one major weakness—a piece of mineral from his home planet—was created fully formed from the writers of the Superman Radio Show in 1943 and first appeared in the Superman comic books in 1949. It tells us that Superman never learned that he came from an alien planet—nor that said planet was named "Krypton"—until 1949. And it also tells us that Lois Lane never learned that Superman and Clark Kent are one-and-the-same until sometime in the 1990s.

If Jerry Siegel had his way, all of these events, and more, would have occurred in 1940—but "The K-Metal from Krypton" was never published.