You know the plan near the end of the movie where Marty is supposed to freak out his mom by revealing he's a communist spy, only for George McFly to come in and punch him? Well, in the original script his method of scaring her is by (and I'm quoting) "feeling her up" in a car. Of course, that doesn't happen ... because Marty's mom takes a shot of booze and it's her who kisses him. On the lips. Even full-blown porn sites delete incest role-playing. To be fair, she does feel extremely weird about it afterward, as would anyone else still left watching this film.
Except Crispin Glover, who'd be sporting a massive boner.
After five weeks shooting this version of the movie, director Robert Zemeckis and Steven Spielberg looked at the footage and realized something was off. The answer was obvious: it was the sexy mom scenes. Perhaps this was exacerbated by the fact that Lea Thompson (Marty's mom) had a bit too much sexual chemistry with Eric Stoltz (Marty McFly), as evidenced by the fact that they later did Some Kind of Wonderful together.
They almost recast the part of Doc for the exact same reason.
Can you imagine Back to the Future becoming the cult classic it is today if they'd kept the incest? It'd be like some shameful movie only perverts admit to owning, like the entire filmography of Woody Allen. While we're on the subject, when the hell are they making Back to the Future 2? C'mon, Spielberg, you've probably made your money back by now.