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.
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.