Deliberate mistake: At the end fight scene, when they are in the barn one of the men falls crashes through the wall when they get hit, spilling the fight to the outside. Unless the barn's walls were made of cheap plywood, the wall would not have given away so easily. Obviously done for dramatic purposes.
Continuity mistake: When we see Clyde demolishing Patrick Scarf's Cadillac, we see Clyde tapping the roof of the car. In the next shot from inside the car, the whole front of the roof is being caved in, but when we see Clyde tapping on the roof again, the roof is back to its original shape.
Visible crew/equipment: When Clyde is demolishing Patrick Scarf's Cadillac, he is using both hands to hold up the front windshield. In the next shot, you can see a crew member wearing an ape glove to help Clyde move the windshield.