Liar, Liar, Slutty Dress on Fire - S2-E2
Revealing mistake: When the cops find the body in the hotel room, the actor playing the body is visibly breathing when he's supposed to be dead.
Escape from the Happy Place - S4-E5
Revealing mistake: When the coach is killed by the monsters, the blood splatters onto the camera lens. The blood stays in one place until the camera slightly pans and it moves with the camera.