After Griff calls Marty a chicken in the cafe 80's and Marty pushes him and everyone starts chasing everybody on the hover boards... if you look closely you can see the wires that were used to make the actors hover coming out of the shoulders of each actor.
When they made this film it was very difficult to get rid of the cables by erasing them out with a computer or something not to mention expensive. The technology wasn't as good then as it was today either. This isn't a slip-up, the director decided to leave the cables in to save money if you listen to the comentary on the special 3 box dvd set of all the back to the future movies, he actually mentions this himself!
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