i thought you were going to say the error was in regards to the darkness.... in s7, death told the boys and cas that before God created angels he created leviathans....and he locked them away in purgatory, that he created, so not as to eat the petri dish....
now death says that angels battled the darkness...it was the great battle before God created earth...
So God entrusted Lucifer with this precious key... the key that holds back THE DARKNESS!!!! or the greatest evil ever known and then DOESTN'T take it back again to give to someone more trustworthy when Lucifer is banished to hell as punishment for not bowing down to man? What, God just FORGOT that the person he was condemning still carried around the most powerful key to the biggest lock in creation? Give me a break! Presumably the Mark was transferable to another person/angel/whomever in the first place so that it could always be taken care of by the most trustworthy of God's allies. So why wasn't Lucifer stripped of the key when he fell? And Lucifer can inexplicably either give the mark away entirely or just share it? Why couldn't Cain do the same? So, did Lucifer GIVE the mark to Cain in it's entirety or just share it? If Lucifer still has the mark then why was THE DARKNESS!!!! released? The whole idea is so riddled with holes that I can't even begin to take it seriously. How much thought did Carver give this idea? Did he trouble shoot it with anyone to see if it holds up to reasonable logic? We've punched so many credibility holes in this idea in a matter of hours, so my guess is NO he didn't even begin to think this through.
I realized this yesterday and holy crap it's a doozy. In the season finale Death says that he can't remove the mark because it would break the seal and let the darkness free. And yet in episode 10-12 "About a Boy" Dean is turned into a teenager and the mark is gone. So the darkness should've been released when the witch's spell was cast on Dean.....oops O_O