I remember seeing on One Tree Hill (not a movie, but a mistake that makes me giggle) when Nathan drove a racecar into a wall, there was actually nothing in the care when he did it, and it was very visible. No dummie or anything. Just. An empty car. And then when his brother, etc. came to his rescue, he was magically back in the car again! =D

Also, on the movie Stand By Me, when the kids are running (cant' remember why, haven't seen it in years) one of the actor's microphones comes dangling out of his trousers as he's running and falls on the floor