Continuity mistake: During the car chase scene, the minivan driven by Brad sometimes has its left tail light and sometimes does not. This mistake is visible several times during the scene.
Other mistake: When Jane is driving with John on the roof, she jumps out when John breaks through the window. The car then proceeds to accelerate through the trees and into the air. The road is not downhill and Jane didn't place anything on the gas pedal.
Answer: She notices that he shakes his right leg twice after peeing, just like her husband.