Okay... here's a good Back to the Future question:

When Doc Brown and Marty travel to the past and then back to the future, the changes they make affect the future they return to. So, in the first episode, his parents are "cool" when he returns.

In the second, however, when Biff steals the time machine to give the almanac to young Biff, he returns to the same future he left from! Shouldn't the future he returned to have been altered to account for young Biff having the almanac?