Plot hole: When Marty lies on the conveyer belt on the way to the stone crusher he tries to wiggle off the belt. The little he seems to be able to move is not justified by the way his hands and feet are tied up.
Revealing mistake: When Jack and Marty drive the skinny lady to her inbred folks Jack follows one of them into his house. He has a terrarium where a snake is just swallowing a screaming but otherwise lifeless rat.
Continuity mistake: After Lionel Pond, the mine owner, dies from a heart attack he sits in his wheelchair with his head sunk deeply between the shoulder pads of his jacket. In the next shot he is sitting straight.
Revealing mistake: Jack pierces the snake owner's hand with his pen with the hidden blade. In close up you see that the hand is a prop: it has a different colour, and there is already a mark where the blade is about to go through.