Double Dukes - S4-E2
Continuity mistake: If you look closely during the first scene where Turk and Moody (disguised as Bo and Luke) reveal to Roscoe that they aren't Bo and Luke, as they peel off their faces. First Moody (as Luke) is wearing a solid light-blue shirt. When we get a close-up of his face, he is wearing Luke's traditional blue-pladed shirt, and after their masks are totally off, Moody is wearing the solid light-blue shirt again.
The Great Bank Robbery - S4-E6
Revealing mistake: The bandits lock Bo and Luke inside closed coffins and then leave. Bo and Luke begin "rocking" the coffins back and forth to tip them over so they can escape. However in the following shots, as Luke is rocking inside his closed coffin, the camera slips or shifts too much, and you can see the barns' interior at the sides of the screen, alongside some equipment.
10 Million Dollar Sheriff (1) - S4-E8
Continuity mistake: In the first scene that Steele is chasing the Dukes in, he hits the side of the General Lee. You can see that the side is badly damaged. But in the next shot when the General Lee is taking a sharp turn and sliding, you can see that the car is repaired.
Dear Diary - S4-E20
Visible crew/equipment: Here you can easily see a support rod and strap at the rear of the General Lee as it holds the camera.
Dear Diary - S4-E20
Continuity mistake: While it's forgiven that the Generals usually have subtle changes through episodes and the series, in this episode we see a General with extremely inconsistent graphics when Bo and Luke deliver the counterfeit money to Winston. The 0 is off and the bottom of the 1 is extremely long.
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