Star Wars Episode 4: A New Hope - Millennium Falcon Set Fails Easily
In the part when Luke, Obi-Wan, Han and Chewbacca are running from the stormtroopers on Tatooine, Han starts up the Falcon. Almost as soon as he starts it up, it fails and 'turns off'. Han then hits it and it turns back on. THIS WAS NOT PLANNED. The Millennium Falcon set electrics must have failed, and when Harrison Ford hit it, they actually decided to keep it in the movie.
Any version of Star Wars Episode IV