Sliptalk has a fun story on a little noticed pair of scenes in the first Back to the Future
movie. I certainly never picked up on it but it is a nice bit of attention to detail from the movie makers. Sliptalk has the low down on the story but it can pretty much be summed up in one image.
The reason for the change was Marty running over one of the original pines in the Delorean back in 1965.