In the scene in The Chamber Of Secrets, with the Whomping Willow, where the blue Ford Anglia crash lands into the tree, the tree bashes the car with Harry, Ron and Hedwig inside and causes several smashes and cracks in the windscreen.
As the car crashes to the floor, falling out of the tree, the car bounces against the ground, and then the camera cuts to a close-up of the car, showing the inside of the car and the back window with the gaping hole. However, the front windscreen should have had a constant light on the window and on the actors, but it doesn’t. No light is on, causing everything to be dark for a second or two, until someone finally has the light turn on…creating a weird flickering light from an unknown light source in the world of Harry Potter. Maybe Harry or Ron secretly used the Lumos spell. Either that, or the crew member blatantly holding up Hedwig, in the back, cast it.
The cracks on the windscreen aren’t there at first, but then flicker on and off momentarily, before displaying properly. Seeing as in one frame Harry and Ron’s faces are in silhouette and then in the next, there is a light on their faces, presumably meant to be from the moonlight outside, so the cracks in the window could be due to the light reflecting against the windscreen, causing the cracks to light up. The only other explanation is that the action shots of the car moving and the static shots of the actors in the car with a light to keep their faces lit, were spliced together, using a flickering effect to merge the two more effectively. Check it out for yourselves in this clip