When Dean is talking to Rufus in his house, the cork in the whiskey bottle disappears from the bottle a few times between shots.
Bela made her 10-year deal in the UK which would come due at midnight Greenwich Mean Time (GMT), which would have been at 7 PM in Erie, PA, not midnight Eastern Standard Time or 5 AM GMT the next day.
The victim who gets his heart removed is wearing a heart rate monitor watch which beeps and shows his heart rate. For these types of watches to accurately portray someone's heart rate, they must be synced up to wet electrodes constantly touching the body. This watch has a rubber strip with electrodes on it that you moisten then fasten around your chest. When the victim's heart is being removed, he is shirtless, revealing that there is no rubber strip fastened to his chest. Yet the watch is still showing his heart rate. This would be impossible without the rubber strip.
Sam says "Maggots...They eat bad tissue and they leave good tissue." This is not entirely correct: Certain types of maggots (such as bot flies) will eat living tissue as well as dead tissue.
The lines "I'll treat you very, very gently. You won't feel a thing. I promise," are not being said by anyone, just captions.
From 09:39 -It is highly unlikely a doctor would operate on himself numerous times yet the brothers are proposing just that.
The guy with the heart rate monitor watch is wearing it backwards/upside down, likely done so the display could be read normally as the camera panned by it.