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: Throughout the film, especially inside the spaceship when its flying, the boy's hair goes from short to long in fact he is at least 6 months to a year older facially too.
Continuity mistake: As David and the ship are leaving the NASA hanger, the straps that are holding the ship down start to break. From the shot outside the ship, you can plainly see the straps hanging on the front off the ship, but from inside the ship looking out the front "viewscreen" you see a clear picture, with no straps obstructing the view.
Continuity mistake: After David has relieved himself, he and Max start a heated conversation about why the computer hadn't returned David back to his own time; a herd of cows grazes nearby. Towards the end of the debate, a cow can be seen near the ship's hatch in a close-up; in a wide shot, the cow is gone; and in a second close-up, the cow stands near the hatch again.
Continuity mistake: When Dr. Faraday and the radar technicians are monitoring David's ship as it free falls towards the U.S. west coast, take a look at their glasses and you'll notice how a map of Japan is being reflected instead (needless to say, the exact same image is reflected three minutes later when the ship enters Tokyo airspace). (01:04:30)
Continuity mistake: When David is in the ship at the NASA hanger, just as Dr. Farraday asks him to get out of the ship, the door closes. From outside the ship, the interior is pitch black, but in the very next shot from inside the ship, it is lit up from the front "viewscreen" and various lights on the wall, which should have been visible as the door closed.
Continuity mistake: The three-digit code on the police car which takes David to his parents' new home changes. Just before it parks by the house, it makes a left turn near the highway. Here we can see its number is 442. But when Detective Banks exits the car, the number is partially visible - with the last two digits being 51 instead of 42. (00:14:30)
Continuity mistake: When Dr. Faraday is talking to David and his family along the New River in Ft. Lauderdale about going to the NASA facility, in the beginning of conversation up until when David says "no" the Andrews Ave. draw bridge in the background is down. When Dr. Faraday convinces David to go to find out where he's been 30 seconds later, the bridge is open/up and is closing/going down during the rest of the conversation. (00:27:30)
Continuity mistake: Right before David sets out to meet up with Jeff in the woods, he's out on the front lawn feeding Bruiser. It eats everything on the tray, save for a couple of pieces - but when Helen calls for David from the patio in the following shot, the tray is nearly filled with dog food all of a sudden. (00:07:10)
Continuity mistake: When Jeff hands David an old flyer, David's feet instantly reposition themselves from the hospital bed's center to edge between shots. (00:19:20)
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 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.
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)
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