Did The First 'The Flash'/'Supergirl' Crossover Foreshadow "Crisis on Earth-X?"

With supervillains descending on Barry Allen and Iris West's wedding this November from the Nazi-dominated world of Earth-X, some fans are wondering how long producers for The CW's DC Universe shows have had the idea for a "Crisis on Earth-X" in their back pocket.

In a scene from the very first crossover between Supergirl and The Flash, Barry Allen struggles to explain the multiverse to members of Team Supergirl.

Like Harrison Wells had done for him, he pulls out a whiteboard and begins diagramming the idea, explaining the differences between various worlds in the multiverse.

One notable example was a world where Nazis won the second world war -- which comic book fans even then recognized as Earth-X.

It would be difficult to imagine that such an example was pulled out of thin air with no regard for the fact that, in the comics, Barry has done battle with the forces of Earth-X numerous times over the years. Doubly difficult when you realize that less than a minute later, Winn and Barry joke about the evil metahumans of Earth-2.

That said, it is almost equally difficult to imagine that anybody at The CW knew that they would eventually do "Crisis on Earth-X" as a story, and planted a clue in a 2016 TV episode so that everyone would be impressed and excited two seasons of TV later.

That said, it is a neat moment -- one that gives a little nod to the comics that would later inspire the big event currently filming in Vancouver and slated for a debut in November.

Supergirl returns on October 9 at 8 p.m. ET/PT on The CW. The next day, The Flash returns at 8 p.m. ET/PT, followed by the season premiere of DC's Legends of Tomorrow at 9 p.m.