There's one major inconsistency: the fact that when Marty travels into the future, his older self is there. That's impossible! If someone goes in a time machine and travels ahead 30 years, then that means that they would be "missing" for those 30 years. THere can't be 2 Marty's. Flight of the Navigator got that right. The kid's in a time machine for a few minutes and is returned 8 years in the future. He hasn't aged but to the rest of his family he was gone 8 years. Duh.....