Plot hole: When Dex is doing the jack in the box ,he is hanging out the sun roof driving with his feet, using the cruise control. When he is decapitated, the body falls and the car spins out and stops. The car's cruise control was still on. The car should have kept going until the brake was pressed. Even if the body hit the brake on the way down, it would have coasted to a stop and not spun out and screeched to a stop.
Other mistake: When the guy's head is cut off, his arms are up in the air next to his head. Yet his head is sliced clean off while his arms remain intact. And they would not have gone slack and fallen down after his head was cut off because they are right next to his cheeks (and the car was moving way too fast to allow for that anyway).