Position of Griffin's tie while he's breaking up with Bonnie.
(at around 25 mins) When Griffin is looking up Kahane's name in a registry, he finds 'Kalchime' on one page and then turns to the next page to find 'Kahane', despite it being an ostensibly alphabetically arranged book.
Although it has better shock value to see and hear the rattlesnake only when Griffin opens the box, but in reality the snake would be rattling fiercely long before he lifts the lid. Rattlesnakes respond to animal heat, not to sight or sound, and Griffin's mere presence in the car would've set it off the moment he got in.
When Mill reads the newspaper story about the murder, a closeup of article reveals that it is just the same few paragraphs printed over and over.
Mud facial dries much too quickly for the few seconds we see it especially on Griffin's face.
Twice in this movie there is a rattlesnake that is heard rattling even though we can see that it is not shaking its rattle.
Crew member reflected in the windows of Griffin Mill's vehicle as Griffin pulls over and kills the snake.
When Griffin opens his door to get out and kill the snake, you can see the entire crew, camera, and lights reflected in the door.
(Possible deliberate error by filmmakers) In the movie-within-a-movie "Habeas Corpus", as Bruce Willis races down the hallway to save Julia Roberts, the boom mike is visible.
Leg and sneaker visible reflecting in the grille of Griffin's Rolls Royce.
In a tracking shot during the confrontation between Mill and the screenwriter, the camera and cameraman are reflected in the black paint of the SUV.