Dave always wears a suit to the office, so we know it wasn't the weekend.
How about this one: in Back to the Future 3, when Marty goes back in time to rescue Doc, he rips the fuel lodge thing and runs out of gas, right? So the whole movie ends up being about how to get the car up to 88 miles an hour. Why didn't they just get the gas out of the other Delorean? There should have been two version of the car at that point:
- The one that Doc involuntarily gets sent to 1885 in.
- The one that Marty uses to travel back and rescue him.
Or how about this one... how did Doc get sent back to 1885 in the first place? He got struck by lightning, but the car wasn't going 88 miles an hour... I think I read somewhere that the lightning strike spiraled the Delorean up to 88mph. That's why you see the car disappear, with circular smoke streaks in its wake.