Revealing mistake: Dean borrows Charlie's phone to call Sam. On the phone, the lock screen can be seen, showing that he never entered in any number.
Continuity mistake: During the scene when Sam and Dean enter the abandoned shipping warehouse to save Charlie, you can see Charlie's legs crossed while sitting in one shot, but in the next shot they are uncrossed. (00:27:10)
Continuity mistake: In the opening scenes, Vamps drags Chrissy Chambers through the shattered passenger-side window of their crappy vehicle, but by Sam and Dean's arrival, that window is magically unbroken. (00:02:30 - 00:15:55)
Taxi Driver - S8-E19
Continuity mistake: When Crowley comes to the ship and finds Kevin Tran he blew out the glass on the ship to get rid of the sigils. When Sam and Dean get to the ship to check on Kevin, and they notice he's gone, all the glass isn't broken and the sigils are still there.
What's Up, Tiger Mommy? - S8-E2
Visible crew/equipment: You can see the crew member's leg go up in the air, and he catches Sam as he runs down the stairs. (00:36:00)
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.