In the last scene of the final episode of season one: Devils Trap...Sam is driving with Dad in the front seat and Dean in the back. As the camera switches from Sam talking to Dad talking Dean (who's supposed to be half dead at this point and not up to moving around much) can be seen sitting up in the backseat behind whoever is talking at the time. Then at one point he's shown slumped down against the side of the car on the passenger side then immediately after he's sitting up behind the driver.