When Cain transfers his mark to Dean it visibly disappears from Cain's arm. Then when Cain teleports Dean and Crowley to safety and rolls up his sleeves to fight the demons, the mark is clearly noticeable on his arm again.
In the summary of the beginning, Sam says things that he didn't say in the same scene of the previous chapter.
Dean looks at the picture of Cain's wife twice, and it clearly says "Colette" on it. Which is pronounced "call-et". But when Dean says her name out loud, he says "cool-ot", like the clothing item "Culottes".
While the viewer could have easily assumed the First Blade would be found somewhere in the contiguous 48 US states since that's where the show takes place for scenes on Earth (barring one scene in season 6 that took place in Scotland), the characters could not have known for certain it would be in the US at all, so when they used the locating spell they should have used a world atlas, as opposed to the map of the US they used, because it would have been a waste of spell ingredients and time had the blade turned out to be anywhere outside the US.
The demons possessing the Confederate soldiers attempt to stop Cain by shooting him with their muskets. Yet a demon should know that would do absolutely nothing to Cain, a bullet barely even slows down a normal demon. It does absolutely nothing against a Knight of Hell, and Cain is even more powerful than the other Knights.