The Godfather: Part II (1974)

Revealing mistake: In the opening scene you can see Paolo breathing, despite him being dead and all. [If you have trouble seeing it, watch the shadow on his back move up and down as he breathes.] (00:03:05)

Revealing mistake: In scene at the brothel, with the senator and the dead prostitute, the (dead) prostitute is breathing in the last shot of her, covered in a bloody sheet. (01:15:30)