One detail in Stranger Things season 1 only makes sense because of Vecna. Stranger Things season 4 introduced viewers to Vecna, a powerful psychic revealed to be the show's true villain all along. Banished to the Upside Down by Eleven, Vecna, a.k.a. Henry Creel and Number One, lies at the center of its hive mind. He is the one responsible for every evil that has befallen Hawkins over the years.
Vecna is an obvious retcon, albeit a smart one that can be woven into the history of Stranger Things without any apparent contradictions. The Duffer Brothers have confirmed they had planned to use Hawkins Lab's Number One as the principal antagonist since beginning work on season 1 — which is why comic book writers were allowed to develop other Hawkins children but not to go near Number One. This explains how Vecna fits so well with the overarching narrative, which had literally been written with gaps in mind for him to fill.
Curiously, though, one specific detail in Stranger Things season 1 only makes sense because of Vecna. According to Vecna, the Demogorgon in Stranger Things season 1 was essentially an advance scout, sent through the gate to explore Hawkins Lab and then the surrounding area. It was seen by Will, and it presumably went after Will either to kill the witness or to bind him into the hive mind instead. Will avoided all the classic horror movie mistakes, rushing home and even bolting himself in. This led to a chilling scene in which the bolt slid open, an apparent display of telekinesis that the Demogorgons have never shown since. The most logical explanation after Stranger Things season 4 is that Vecna was watching over the Demogorgon psychically and used his own telekinetic powers on that bolt.
How Vecna Influenced Stranger Things All Along
Eleven closed the first Hawkins gate at the end of Stranger Things season 2, proving herself to be even more of a threat than Vecna had realized. When the Russians opened a new gate, Vecna decided she was the one he needed to deal with; all of Stranger Things season 3 is best understood as his counterattack, with Billy chosen as the most important of the Flayed simply because he could serve to draw Eleven's psychic attention. Vecna's plans again failed, with Hopper and Joyce managing to destroy the Russian Gate before he could either bring Eleven into the hive mind or kill her, but presumably, Vecna had deliberately caused an injury that cut off Eleven's powers. With Eleven hopefully out of the way, Vecna then began trying to figure out a way to create his own gates, tired of using ones that kept being shut down from the other side. All this strategy is an obvious retcon, but it works surprisingly well, weaving a single narrative through the entire Stranger Things story.