The Great Bank Robbery - S4-E6
Revealing mistake: The bandits lock Bo and Luke inside closed coffins and then leave. Bo and Luke begin "rocking" the coffins back and forth to tip them over so they can escape. However in the following shots, as Luke is rocking inside his closed coffin, the camera slips or shifts too much, and you can see the barns' interior at the sides of the screen, alongside some equipment.
Return of the Fighting 69th - S1-E6
Revealing mistake: Before Roxanne Trent hits the box, it is possible to see that her hand is normal, not iron. (00:17:01)