Sin City (2005)

Revealing mistake: When Miho plugs the barrel, Jackie boy tries to fire, but when he does, the slide fires into his skull area. Notice that the barrel is larger in front. If that slide was straight, then o.k. But, the head of the barrel is larger at the far end (business end). It is not possible for the larger end to pass through the rest of the gun (focus on the bottom part) without every part of the gun disintegrating. (01:01:55)

ButteredFilm

Revealing mistake: When dead Jackie Boy is talking to Dwight, you can see his normal neck under the fake slit-throat neck, and once or twice, you can even see a little device that was meant to shoot out smoke from a cigarette in the fake appliance.

Revealing mistake: When Marv is starting to dunk the man's head into the toilet, trying to get more information, you can see the rim of the toilet flex slightly from his weight.

Revealing mistake: In some shots towards the end when it's snowing, you can see the snow (computer generated) simply disappear, especially in over-head shots. It isn't hitting the ground, it just disappears.

Revealing mistake: When you see characters in closeups when it is snowing, no snow ever lands on them, because it's all digital.

Deadman63

Revealing mistake: When Jackie Boy's hand is chopped off, you can see some of the blood flying out is CGI and appears to be "flat" and doesn't quite match in with the rest of the shot.

Revealing mistake: When Hartigan is talking to Shellie the waitress, asking about Nancy, you can tell that the bottles of beer are glued or otherwise fastened to Shellie's tray, because she tilts the tray enough to tip the bottles over several times, and you can see the liquid inside them tilt as well, but the bottles stay put.