At the start, Dean is holding Sam's jacket with his right hand and has the knife in his left. The camera turns away and when it comes back he's holding Sam's jacket with both hands and there's no sign of the knife. Then when they make to leave he has the knife again.
Right after Dean activates the sigil that sends the Angels away, he says "I learned that from my buddy Cas" but the first time Dean saw that sigil used was when Anna used it in "Heaven & Hell".
When Zachariah is torturing Dean in order to get him to agree to allow Michael to enter him as his vessel, Dean's bottom lip becomes bloody after he gets stomach cancer. A few moments later, the blood is gone in the close up as Zacharias grabs Dean by the chin, only to reappear in the following wider shot.
Becky has a sign reading "Winchester Boulevard" on her wall, despite the fact that a few short episodes ago Chuck stated that he never once used their surname in any of the novels.
When Sam and Dean visit Chuck, the Angels arrive but Dean has prepared a sigil to send them back to Heaven.
Zacharia notices Dean bleeding and we see a shot of Dean's left hand dripping with blood. But when Dean activates the sigil he does so with his right hand which clearly wasn't bleeding as it doesn't leave an additional bloodstain on the sigil.
When Becky knocks at the door, she says "You're Sam Winchester..." but it was revealed in 4.18 The Monster at the End of This Book that Chuck never wrote the brother's last name in his books. However, we know that Chuck and Becky's video chat continued past the extent of the scene. Since we don't know the full extent of the conversation it can easily be assumed that Chuck revealed the brothers' last name to her - possibly as a means of giving her a way to authenticate herself to the guys.
When Sam and Dean visit Chuck, the Angels arrive but Dean sends them back to Heaven. Dean was not out of shot long enough to create the sigil but he might have anticipated the angels may arrive and he created the sigil before they did.
Bobby being possessed by a demon? P-lease. He has been a hunter way longer than Sam and Dean. He was the one that gave the boys the charms that protect against demon possession in "Born Under a Bad Sign" (S2 Ep14). And we are to believe that he neither wears a charm nor has a protective tattoo? Sorry, but that's too much of a stretch.
In the opening plane scene, the pilot mispronounces Ellicott City, Maryland. It's pronounced ELLA-kit, not ella-COT.