Swamp Molly - S1-E6
Revealing mistake: When the truck goes into the water, you can see the driver was either a stunt driver or a dummy. Also, you can see the top of the truck just under the surface of the water, suggesting it was on the bottom. A few seconds later, and all you can see are bubbles.
Revealing mistake: When the boys are stopping to check the ambulance, you can see the smoke device burning used to simulate the engine running badly.
Revealing mistake: When spying on the hidden chop shop, the boys are using Cooter as a stepladder, with their feet on his shoulders. After he complains about their boots, they jump down after Daisy warns them. If you look at Cooter's shoulders, you can see that they're padded for the stunt.
Daisy's Song - S1-E2
Revealing mistake: As the Sheriff's cars are nearing the area where the Mobile Madam is, they're driving fast. As the RV and Bo are nearing the gate of the property they're on, you can see the two Sheriff's cars sitting just to the left of frame, then just beginning to move to make the pursuit look good.
Revealing mistake: Three times during the final chase there are close ups of the boys in the General Lee. If you look, the car is stationary on a brown dirt road, and the car is being shaken to simulate driving.
Revealing mistake: When there is a close up of the boys the first time they are driving through the junkyard, you can see through the passenger window that the car is being shaken to simulate driving.
Revealing mistake: When Turk pushes Bo onto the tires, you can see his stunt double.
Revealing mistake: When the sheriff's cruiser is first seen, you can see Cooter's yellow cap. After the jump, you can see the black helmet of the stunt driver, and the patch of the Sheriff's office.
Answer: Various reasons, but mainly to drive the Dukes out of Hazzard County because the family have been foiling his schemes since his and Uncle Jesse's bootlegging days.
Captain Defenestrator