Continuity mistake: Before the fire scene, the rain gutter is on fire in several places. But when the rain gutter gets disconnected, the fire in the rest of the rain gutter mysteriously disappears.
Continuity mistake: In the 'race scene' near the end of the film, just before Greg and Jack go their separate ways, you see Larry (who is sitting next to Greg) say, 'Oh, look, there's Jack. Hey, Jackie,' and he waves. Then it cuts to Jack, who is looking at Greg, and then it goes back to Greg, and in the background you see Larry and he is unfolding a newspaper and starts reading it. Then it goes back to Jack, and then back to Greg, where you see Larry with his hands on the dashboard, looking at Jack. So what was with the newspaper? Where did it come from? How did it disappear so quickly?
Audio problem: When Jack is talking to Greg inside the changing room, Jack is saying, "I'll be watching you", but his mouth is still moving saying something else. (00:51:40)
Plot hole: The fake Jinx wouldn't be able to get into the den, the door was closed.
Continuity mistake: During the scene when everyone is gathered around discussing the whereabouts of Mr. Jinx, Jack announces "We'll just have to reschedule..." Debbie jumps up from the couch and when she does she hits the coffee table causing one of the edges of the table to fall and hit Denny's knee. The next shot the edge is back in place off Denny's knee and Denny has slightly changed positions.
Continuity mistake: Before and throughout the scene where Jack demonstrates his hidden cameras, the lemon and cherry in Greg's glass keep moving around.
Continuity mistake: When Greg is trying to open the suitcase, the top pouch is empty, when the delivery man drops it off, and when Jack tries to open it, the top pouch is full.
Continuity mistake: At the dinner table (the urn scene) after Robert De Niro reads his poem he puts it in his wallet and closes his wallet, then in the next scene in the bottom left you see him close his wallet again.
Other mistake: Near the end of the movie, Greg says he's in row 8 on the plane. But when he's on the plane, we see there are only about 7 or 8 rows behind him. If the plane had only 15, 16 rows, the plane wouldn't have that middle section of seats.
Other mistake: When Greg asks the kid behind the counter if they have the Nicorette patches, he says no, and hands him a pack of the Nicorette gum. However, the package to the left of the gum, and possibly to the right, are patches.
Continuity mistake: Robert De Niro's hairstyle changes throughout the film. At some points it's thin on top, at others it's rather thick.
Continuity mistake: When Pam is ringing Greg near the end, Jack is watching from the side, and it is obvious how much longer his hair is compared to the rest of the movie. His hair is quite short in the rest of the movie. So it is obvious (since he looks younger in that scene) that that scene had been shot near the start of the filming.
Factual error: Ben Stiller buys Champagne in a drug store, yet in NY the only legal place to buy wine/champagne is a liquor store.
Factual error: The opening scene outside the pre-school is supposed to take place in Chicago, but actually is somewhere in New York State you can tell by the postcard-like registration sticker in the lower driver side windshield of the cars.
Continuity mistake: For the first half of the movie, the cat's tail changes colour from black to light brown, to black again.
Continuity mistake: When the characters are standing in the den after the cat has ruined everything, there are plates all over the floor. There is a "T-shaped" pile of plates around De Niro's right foot that almost rotates between cuts.
Continuity mistake: During the race scene, when Greg gets going again after stopping for the first red light, Jacks car shoots past him. They then race towards the next red light, but when Greg stops, Jack pulls up from behind, as if he had never driven past them.
Continuity mistake: When Greg is at the airport's lost luggage department you see the baby chuck up on him but you can see it actually comes from too low down. (00:08:00)
Other mistake: When Ben Stiller borrows clothes from Robert DeNiro, he was wearing a beige turtle neck sweater. In the breakfast scene when Owen Wilson is introduced, Teri Polo wears the same sweater for most of the rest of the movie.
Continuity mistake: At the dinner table when Greg is trying to open the champagne, Pam look straight ahead startled for a second, then in the next frame she is talking to her dad again. It looks like she is seeing the urn hit before it is in the film.
Answer: Who says they had to have been legal adults to date when Top Gun was popular? They could very well have been teenagers and got engaged shortly afterwards. If Teri Polo and Owen Wilson are playing characters that were born the same years as the actors themselves they would have been 17 and 18 respectively when Top Gun was released. If they are playing characters a few years older than they actually are, which is entirely plausible, what Pam says makes perfect sense.
BaconIsMyBFF
I would like to add that a movie doesn't stop being "very popular" soon after the release. In 2000, when I was in middle/junior high school, we actively talked about movies that had been released three or more years before (Forrest Gump, Scream, Cruel Intentions, etc.). A movie from 1986 could easily be popular among a dating/engaged couple and their friends in, say, 1990.