Revealing mistake: Whenever someone takes drugs and their pupils "dilate" you can see that it's a normal eye being distorted by a computer because the arc of reflected light also is distorted.
Revealing mistake: Near the end of the movie, when Malcolm's partner pulls out his gun and Lester shoots him, the bullet hole in the wall appears before the bullet wound in Paul Giamatti's arm.
Revealing mistake: When Ed falls on his belly and dies of a heart-attack at the end of the film, if you look closely, you can see the actor is still breathing a few times. He's trying to hold his breath, but every so often, you can see his back subtly raise and lower as he slowly takes a breath in and out.