All Hell Breaks Loose: Part 1 - S2-E21
Continuity mistake: Before the Roadhouse fire, Ash and Dean are talking on the phone. Ash looks at his watch, on his left hand, it says 8:10, and has a black dial/face. As Dean and Bobby go through the rubble, Dean identifies Ash by his watch. Still on his left hand, it is now upside down, evident by the crown of the watch (used to move the hands of the watch) which previously facing his hand, is now facing his elbow. Also, the dial/face of the watch is now silver instead of black. This is a different watch, and was inadvertently put on the burnt arm upside down.
Hell House - S1-E17
Visible crew/equipment: When Dean, Sam, Ed and Harry have just shot Mordechai, the cameraman's arm is visible on the right-hand side of the screen. (00:33:30)
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)
Continuity mistake: Sam claims it is 12am and the the bug storm is on its way, but five minutes later the sun is shining. (00:37:00)
Factual error: When Sam and Dean show up at the crime scene to investigate, they are stopped by a woman wearing digital Army ACU's. She introduces herself as Sergeant Miranda Bates, even though she has rank insignia on her cover and her chest that identifies her as a PFC (private first class). (00:14:05)
Hell House - S1-E17
Revealing mistake: Dean gets his phone and snaps a picture of Sam when he's asleep. You see his phone take the picture but the screen is then white, showing he didn't really take anything.
Visible crew/equipment: When Sam and Dean are talking to Bobby about to leave after exorcising Meg, in the bottom right corner of the screen a cameraman's arm and camera can be seen. (00:17:10)
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.
Born Under a Bad Sign - S2-E14
Revealing mistake: When Dean turns over the deceased man lying on the floor, the man blinks while Sam and Dean are arguing about whether or not Sam is responsible for his death. (00:10:25)
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: When Dean reads about the Roosevelt Asylum in his dad's journal, there is a picture of a Wendigo on the opposite page of the newspaper clipping but in a different scene, Dean checks the article again for information on the South Wing in the Asylum but this time on the opposite page of the newspaper clipping, there is no picture.
Fresh Blood - S3-E7
Revealing mistake: Dean cuts himself near the beginning of the episode, however the blood begins to drip (spurt really) before the blade cuts into his arm.
Plot hole: When Sam and Dean realise they need to destroy the hook to stop the Hook Man, they find out that it has been reforged into something else. They state it needs to be set on fire to stop him, but if it was melted down and reforged then it has already been set on fire and the Hook Man shouldn't be latched on to it in the first place.
Factual error: When Sam and Dean are showing Jo pictures of H. H. Holmes' victims, the last picture was a victim that was not his. The final black and white is Jack the Ripper's third victim, Elisabeth "Long Liz" Stride.
Provenance - S1-E19
Audio problem: When Sarah goes to touch Evelyn's shoulder, you hear Sam's voice telling her to stop, except his mouth isn't moving at the time the phrase is being uttered. (00:23:50)
Continuity mistake: The baby gets into the fridge, which then closes - we see a closeup shot of the babyproofing clip shutting itself. Then when the mother sees the spilled milk and realises what's happened, there are a few shots of the fridge - in shots from the front the clip's open, and in shots from the side it's closed. When she goes to open the fridge it's open again - she even fiddles with it to give the appearance of undoing it before opening the fridge door.
Continuity mistake: Near The End of the episode when Dean is being chased by the possessed truck, Sam tells him to take a left. After he's made the left, Sam tells him to go exactly 7/10ths of a mile and stop. Dean looks at his gage and it reads 70098.3. Dean looks up, then back at it: it changes to 70098.5, but the next time he looks down it says 70100.6 and when he stops it says 70101.0, meaning he's gone 2 miles too far. (Probably meaning it took two shots to get the scene right but they messed up on the gage.) (00:35:00)
Visible crew/equipment: In the scene before Dean makes the promise to Sam, as the camera pans along the side of the Impala, in the rear of the car, you can see the dolly track that the camera runs along.
All Hell Breaks Loose: Part 1 - S2-E21
Continuity mistake: In the scene where Dean is with Bobby at the burned down Roadhouse and Dean gets Andy's vision about where Sam is, the first time Dean scrunches his face in pain he puts his left hand to his head. The shot changes to a back view and you can tell his left hand is still at his head. But when the shot switches again Dean is bringing his right hand down from his head.
Bloody Mary - S1-E5
Continuity mistake: When Sam and Dean enter the Shoemaker home, a woman leaning against a wall holding a coffee cup starts to follow them down the hall. When the shot changes, the woman is back against the wall, talking to someone who wasn't there in the previous shot. (00:08:00)
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.