Supernatural was created by Eric Kripke and premiered in 2005. It was initially planned for 3 seasons, expanded to 5, and then grew to fifteen. Over the course of the show, while Sam and Dean stay as our anchoring protagonists, the cast and world expanded into a sprawling complex universe.
Currently, Supernatural is on its fifteenth and final season with an impressive run of over 300 episodes. The show's longevity is thanks, in part, to its dedicated fan-base. A fan-base that the show itself acknowledges in-world, perhaps most famously in the season 10 episode: Fan Fiction.
The show has also had a large impact in pop-culture. Having been referenced in everything from comics, books and TV shows, to a tweet from the fast-food chain Arby's featuring an anti-possession symbol made out of ketchup.
Supernatural's blend of complex over arching stories and meta, self-referential humor leaves a lot of room for details that viewers might not catch in a first watch. With so many episodes, it's hard to blame you, so kudos to any true-blue fans who caught these all in a first viewing.
10. Negan's Bat
Dean and Sam have been known to use a wide variety of weapons while hunting, but in Season 12, episode 15, 'Somewhere Between Heaven and Hell', we see something new: a baseball bat wrapped in barbed-wire. A weapon some fans recognized as Lucille from Walking Dead.
Mid-conversation with Sam, Dean references the bat and casually says, "Dad loved this thing." An easy catch for fans of both The Walking Dead and Supernatural, but not everyone noticed the quick reference on a first watch.
In Walking Dead, Sanctuary leader Negan is played by Jeffrey Dean Morgan... the same actor who played Dean and Sam's father, John Winchester, in Supernatural.
Jensen Ackles also tweeted a picture of Dean holding the iconic weapon with JDM playfully tweeting back, "You weren't supposed to find that. Gonna need her back son. I mean... I already gave you my car. And? I don't know? Sorta, died for you."