Revealing mistake: In the scene where Norton's wife is target shooting with the revolver, there aren't any bullet heads showing in the cylinder. There's also no recoil as she fires.
Revealing mistake: When Will's wife shoots Dollarhyde at the end, watch closely when she shoots him for a second time and hits him in the cheek. For about one frame, you can see what looks like a wire coming out of his cheek that has caught some light, and about a half-second later, you can see some sort-of small brownish cap hit the floor on the right side of the screen. (It's very tiny so you have to look closely.) These types of effects are usually accomplished by putting a small prosthetic cap attached to a piece of wire over the bullet wound makeup, which is then pulled from off-screen to reveal the "bullet hole." It looks like both the wire and prosthetic cap both managed to get caught in the shot. (Slow motion or freeze-framing help but aren't required - both the wire and the cap are easy to catch once you see them the first time).