Revealing mistake: When David enters the food robot to escape, he makes a left at a cop car with the license plate 11385. A few moments later, he passes the same exact car going in the other direction. (00:41:40)
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)
Character mistake: Max tells David that humans only use 10% of their brain. That's a myth.
Continuity mistake: When David is using a map and eating a candy bar, one shot shows it bitten into. The next shot shows it whole, and the next shows it bitten into again.
Continuity mistake: When the car full of "geeks" takes off from the intersection, the white semi that was approaching should have met and passed the car at the speed it was travelling, but disappears.
Revealing mistake: When introduced to what David calls "Popcorn", the wires moving the puppet are visible.
Other mistake: When examining the craft in the hangar, it's reported there are no seams or any way in or out, but when the camera focuses on the man's hand, you can see the seam where the hatchway will be.
Continuity mistake: When David goes to his room early in the movie (in 1978), if you look closely you can pick up what looks like a football card wrapper on the shelf on the left side of his room, near the white helmet and behind David's Dad. Comparing to known wrappers, this one was issued in 1985.
Continuity mistake: After David contacts Jeff from a payphone, he heads back toward the ship where a father is getting ready to snap a family photo. Behind the gas pumps is a Dodge Caravan with closed doors. Once the ship lifts off a few seconds later, you can see how the van's rear passenger door has managed to open up all by itself. (01:15:15)
Continuity mistake: Before returning home to 1978, David steps out of the ship to say goodbye. When Bill and Helen first run towards the ship, the front door of their house opens up between frames. (01:20:10)
Continuity mistake: In 1986 when David is reunited with his parents at the front doorstep of their new home, Bill places his left hand near Bruiser at one point. But in the following shot where David faints on Helen's lap we can see Bill's right hand near Bruiser instead. (00:15:45)
Continuity mistake: When the first two guards find a security breach involving David and the ship, guard 2 activates several switches on the wall. Between shots, his left hand places itself on the control panel. (00:47:30)
Continuity mistake: Within moments of being confined to his room at NASA headquarters, David meets Carolyn and R.A.L.F. When Carolyn first pokes her head into the room, her hand repositions itself on the door frame as the shot changes, moving closer to the light switch plate. (00:30:30)
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)
Audio problem: After it's revealed that David has been missing for 4.4 hours in analysis mode on Phaelon, one of the technicians says, "Light speed theory" but the words aren't consistent with the movement of his lips. (00:36:15)
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