It's not like the show ever really explained its inconsistencies, but still.
Throughout its 15-year run, Supernatural has had many problems with its internal logic and plot. Sometimes the show would just debunk itself, while at other times it would simply erase convenient tools like the word Cristo as if they never existed.
However, the plot hole count continues up to this day, despite the fact that the show ended back in 2020. For example, it looks like there is a serious problem with the entire Mark of Cain storyline.
According to Supernatural, God first gave the mark to Lucifer so he could imprison Amara. Then he gave it to Cain after he killed his brother with the First Blade. Something just doesn't add up. Redditor Memelordreadscott pointed out that the mark appears to be older than the blade, so their connection is at least questionable.
The show has never really addressed the inconsistency, but this is where fan theories come in. One fan has come up with an explanation that might actually work for Supernatural.
"I think that when Cain killed his brother he somehow channeled some of the Mark's power into the blade. There are several examples throughout the series of how things like murder have a tangible effect on a person's soul. I think in the moment when Cain killed Abel he saw the blade as an extension of himself, and thus some of his power flowed into the blade and changed it forever," Redditor LookWhatYouLearned suggested.
No matter how good the fan explanations are, the plot hole is unlikely to ever be addressed by the show itself, unless the prequel, The Winchesters, decides to return to the storyline.
The Winchesters is back from the hiatus, with episode 8 airing on The CW on January 24.