When Bo and Luke are tied up inside the sawmill and Boss, Rosco and their henchmen leave them, the boys legs are not tied up. When one of the henchmen goes back into the sawmill to take Bo's wallet, the boys legs are now tied up with rope around their ankles.
In the close-up shots of Roscoe and the postal inspector chasing the Chinaman on the motorcycle, the police car's siren can be heard but the lights are not going.
The police car has its lights flashing and slides to a slow gradual stop after smashing through the row of striped barricades, but then in the following close-up shot of Roscoe and the postal inspector in the car's front seats, they both perform a "pitch forward and forcibly lean back" maneuver as if they had come to a sudden stop from a moderate speed, and the overhead lights are not going.
Obviously fake water-jets used in the scene where Cletus skids into the side of the water-truck --- the water starts spraying out about a half-second before the car collides with the truck and supposedly ruptures the tank, also, the car does not hit hard enough to do that much damage to the tank or pipes.
When Cletus, Boss Hogg, and the Postal Inspector lady are riding in the police car, they are hit from behind by Rosco in his car with the hood up. They slide into the side of a water trough. The water is splashed out away from the car but in the next scene, the water is splashed back into the car.