And Then There Were None - S6-E16
Revealing mistake: Samuel is very much dead, laid out on the table in the canning factory employee lounge, but as Sam stands over him, talking about whether or not it was right to gank him, you can see Samuel's chest rising and falling as he breathes. (00:26:30)
Factual error: In the opening, H.P. Lovecraft is calming his nerves with alcohol. In real life, Lovecraft was a complete teetotaler, who even wrote stories about the evils of liquor.
Revealing mistake: As Dean digs into the dead guy's guts and discovers the claw the pale dead guy blinks.
Continuity mistake: Season 6 Episode 15 'The French Mistake': When Sam and Dean are first talking to Misha, the collar of his shirt is tucked into his jacket, as normal. However it suddenly is sticking out over the top of his coat, then changes to being tucked in again, with no time for him to have changed it himself.
Mannequin 3: The Reckoning - S6-E14
Continuity mistake: When the guys tip Rose into the grave, her eyes are open, but as she lands and dirt falls over her face, her eyes are closed. A dead person's eyes don't close on their own. (00:25:20)
Mannequin 3: The Reckoning - S6-E14
Audio problem: In 6.14 "Mannequins 3: The Reckoning", when Dean goes up to talk to Ben, Ben is playing a video game. Dean says something along the lines of "There's no way plants could kill zombies", therefore pointing out that Ben is playing Plants vs. Zombies, but the sounds coming from Ben's gaming system are not from Plants vs. Zombies.
Continuity mistake: When Sheriff Mills and Bobby are talking about transferring Rufus, her lapel goes from being under her seatbelt to over several times in subsequent shots.
Revealing mistake: When the woman is under the bed with her baby her arm is covered in blood, but when she touches the baby's face and clothes, no blood comes off onto the baby.
Answer: Angel blades come from angels. It's their angelic magic that makes it special. Therefore, an archangel blade is made magical with the powers of an archangel. They couldn't have just created one or used one, as none of them are archangels.