Major Plot hole: Doc puts Jennifer to sleep early in the ride to the future, because he doesn’t want her interfering. Marty asks why he brought her in the first place, and Doc says, “She saw a time machine; I couldn’t just leave her there with that information.” Doc is concerned about being inconspicuous, but he just left a residential neighborhood in a flying car that exploded into a fireball. A calculating scientist would have simply driven to Marty’s house, told him the coolest cliché of all time “Come with me; I’ll explain on the way,” and used the DeLorean from a remote location.
Super Plot hole: Doc. Brown comes back from the future and claims that he needs Marty and Jennifer to follow him in order to save their future children. This is one of the most retarded things ever put on a screenplay. If your children are in danger in the future and you go in the future to save them; when you are coming back to the present nothing that you did in the future changed anything. You will still have to save them AGAIN when the present will reach the future moment.
Since the future is not shaping the present (unlike the past) there is no logical reason to go in the future to save your children…because they don’t exist yet. The only thing Doc. Brown needs to do, is to inform them of what is gonna happen in the future so that they can alter it later.
It could also go further then that. If Doc.Brown comes back from the future and informs Marty and Jennifer that their children are in danger; the simple fact of exposing to them that they are going to get married and have kids might destroy the possibility of this specific conclusion to happen.
Super Plot hole: Marty McFly acquires the Sports Almanac 2000 in the year 2015 before Doc realises Marty’s plan and makes him throw it away. ‘Old Biff’ however puts two and three together, takes the Almanac, goes back into the past to give it to his younger self, before returning exhausted to the same 2015. The big problem with this is that we know that changing the past changes the future.
When Doc and Marty go back to 1985 the whole world has been altered by Old Biff’s meddling. However his meddling would have actually changed the 2015 that they were already in – there was no way that Old Biff could have returned to the same 2015. He’d have traveled forwards to the 2015 created by Young Biff’s successful betting scam. Not only would this action have messed up everything in Part II, it would have changed everything in Part I also. (By N. Baseley)