In this movie Doc explains that when you look at yourself in the future or past you will rupture the time continum. This is demonsrated when Doc past meet Doc present and Doc presents hides his face. Also when present Jen meets future Jen and they pass out. How come when Biff future goes back to 1955, he meets with Biff past, and nothing happens? Does Biff have some sort of special machine to prevent that? Or was Doc wrong in his calculations.