The way Sam jabs the blood injections into Dean's arm would have been totally counterproductive, since the blood would not have gone into the vein but would have created an enormous haemotoma or tissue inflammation.
When Sam goes into Dean's room, there is a partially-eaten piece of pie on a side table. Several months have elapsed, in the show's time line, since Dean was turned into a Demon and left the Bunker, so the remaining bit of pie should, at best, have been moldy.
Demon Dean should never have been able to leave the demon trap on the floor in the bunker.
Now that Castiel is up to full power (albeit with borrowed grace), he could easily cure Sam's arm. Since the injury was a real-world injury to Jared Padalecki, it isn't possible for this to happen.
While being held in the bunker, Demon Dean says to Sam about Lester "you took a guy at his lowest, used him, and it cost him his life *and* his soul." However, in the previous episode, Crowley told Dean that he couldn't collect Lester's soul because Dean didn't kill Lester's wife.