So Then a Bat or A Monkey - S1-E1
Revealing mistake: When Joel puts the dead body in the freezer, the eyes are open. When he pushes the head to the side so he can close the lid the body plainly closes his eyes.
Revealing mistake: When Bob and Joyce have lunch, Bob pops open a can of soda (which is period appropriate). The scene cuts to Joyce, then back to Bob. It is clear from how he is holding the can, and how he pretends to drink from it, that his newly opened soda is actually empty.
Revealing mistake: When Bolivar's assistant comes running out of the theater where she found him, there are New York cabs as well as Toronto cabs in the background. The passing ambulances are marked as from Toronto, not New York.
Revealing mistake: When Nathan and Audrey are sitting at the table in The Grey Gull, the extra in the background can be seen bringing a loaded fork inches short of his mouth then fake chewing. This happens a few times before cutting to the ending credits. (00:40:05 - 00:40:55)
Revealing mistake: The woman becomes possessed while the police are attempting to arrest her. She turns her head around and cracks her shoulders so she can walk at them backwards. They did not change her hand positions, so her right hand is where her left should be and vice versa. (00:11:00)
Revealing mistake: In the scene where Mitchell first visits McNair's van, Mitchell's reflection is visible in the window on the left side of the screen during the shot where he first opens the van door. The vampire lore of the show informs the viewer many, many times that vampires (such as Mitchell) and ghosts cannot be seen in reflections, pictures, or videos (their image "cannot be captured"). The reflection can be seen for roughly 20 seconds. (00:23:20)
Revelations - S1-E10
Revealing mistake: When Kieran looks at his phone in the car, the image of Emma calling him is upside down. The image was added after the filming was done.
Revealing mistake: When The Wraith is parked at the convenience store, wet tire prints are seen on the concrete from previous takes of the car backing up and turning off the store's lot. This is confirmed later as Charlie Manx actually reverses the car when he drives off the lot.
Revealing mistake: In the close-up of Frost's hand while the doctor is stitching up his wound, you can see the delineated edges of the latex make-up patch used to simulate the injury. (00:12:30)
Revealing mistake: Multiple times when we see the speedometer of the minivan, the RPMs aren't moving. The car's shifter is also noticeably in park. This is obvious as its higher, whereas if were in drive, it would be lower. Even during the most intense moments, the RPMs never change. The car is obviously being towed by a camera vehicle.
Revealing mistake: The bullets the priest takes out of the ammunition tray have no primers. (01:01:33)
Sicut Cervus - S2-E6
Revealing mistake: When Strand is sitting on the bed with Thomas, Thomas takes his last breath. Seconds later, you can see Thomas swallow.
Beauty Rest - S4-E5
Revealing mistake: Kathy Ireland is supposed to be dead but watch carefully and you'll see her chest move when Minnie Driver's doing CPR.
Revealing mistake: As Fester looks at the mirror, it shatters. After he asks a question, it falls off the wall. After it does, you can see what appears to be someone's fingers pushing from the other side to knock the mirror down. That spot on the wall is obviously fabric covering a hole.
Twice the Grieving, Double the Loss - S1-E7
Revealing mistake: During the flashback, when Chucky is putting on his coat to leave the apartment, if you look very closely, you can just barely see the outline of either a very tight sports bra, or chest-binder top under Chucky's white shirt. (The white shirt is ever-so-slightly translucent in the light.) Since Chucky is played by a woman in these scenes, this garment would be used to help hide her breasts and give her a flatter, male-looking chest.
Revealing mistake: As Sgt. Cain and Chief Lain stand by Marjorie's dead body lying on the hotel bed, discussing her death from an overdose, you can see her stomach rise and fall slightly, though she's supposed to be dead. (00:18:00 - 00:20:00)
Revealing mistake: Just after falling to her death from the balcony, Olivia's corpse adjusts her arm toward her body. (00:53:30)