I've avoided at great length the problems with time travel that exist in the Back to the Future series. At some points changes in the time-line result in a rippling effect that take time to reverberate throughout the ages. In other cases, what happens to one characters past self seems to have no real change on that characters future self.2 Perhaps there exist multiple quantum realities that they're slipping in an out of as they transgress the boundaries of time. The only consistent is that the future is inconsistent, but changes in the past can have devastating ramifications.3
1. Marty: "Whoa, this is heavy." Doc: "There's that word again; 'heavy.' Why are things so heavy in the future? Is there a problem with the earth's gravitational pull?"
2. e.g., Future Doc who was stuck in the past should have known that past Doc who was in his relative future, would send Marty back to help him.
3. Of course, many series have had this problem, including the great Star Trek which Ryan and I (along with a multitude of other nerds) have had many long conversations discussing.