During the introduction of the Aston "Vanish", Q explains that the camouflage is achieved through tiny cameras that project the image they see on the opposite side of the car. In fact, you can even see Q's legs as he walks behind the car.

Now here's the slip-up: Later in the movie, Bond hides behind his invisible Aston so the passing henchmen don't see him. But really, by the way the car works, he SHOULD be projected on the opposite side of the car, which WOULD be consistent with Q's earlier explanation. But he's not, and therein lies the slip-up.

I guess Bond is so awesome he can make his car function how he wants, whenever he wants.