Continuity mistake: In a matter of 15 seconds between David's ship exiting the hangar and shooting 20 miles upwards into space, shadows on the ground indicate very different positions of the sun. (00:51:05)
Continuity mistake: When Carolyn first enters David's room at NASA headquarters, she introduces R.A.L.F. Between shots, David wraps both arms around his knees. (00:30:35)
Continuity mistake: When David and Max are lost and in need of directions, they spot a red Mustang cruising along. The Mustang is within throwing distance of a T junction at first, but then in the following shot it's suddenly about a quarter mile away from it. (01:09:30)
Answer: I can't directly answer this, but I was watching this movie just now, and didn't take in the girl at the beginning's (Jennifer's) name, so when Sarah Jessica Parker, Carolyn, appeared I then spent the rest of the movie assuming that she was going to turn out to be the girl he was crushing on in 1978, especially when Carolyn even told him he was cute (which is kind of a weird thing for a 20-year old to say to a 12-year-old if there isn't a plot reason for it). I do wonder if that was in fact the originally intended idea that was changed for whatever reason before the movie was completed, and if perhaps he was meant to go back to 1978 with the knowledge that she found him cute and therefore a new-found confidence to ask 12 year-old her out.
MrDannyDetail