During the filming of "Back to the Future III" (1990) on December 18th, 1989, a neighbour filmed the production that took place at a house in Arleta, California, which in the film acts as Marty McFly's home (YouTube link).

Universal Studios returns to the previous location in Arleta California used as Marty McFly's house for the 3rd and final sequel of the trilogy. The Delorean Time Machine has already been destroyed by the train and Marty runs back to his house to find old Biff polishing his Dad's BMW and his new Toyota in the garage..... imagine this is all playing out in front of you as you view it from the next door neighbor's house, through the kitchen window on your new VHS camcorder! This amazing behind the scenes footage has been kept on the shelf of that neighbors house ever since that day, only to be given to me (Videobob of Bob's Prop Shop) on my recent visit to the house in March of 2013 to be shared with you the fans!!!

Watch the footage:

Via Boing Boing.