Continuity mistake: Near the end of the film Greg is talking to Pam. She is laying on the bed and he is sitting on the bed . The scene moves back and forth from him to her and every time the pendant on her necklace moves from side to side.
Continuity mistake: Pam's hair constantly looks different from shot to shot in different scenes. For example, when they first get to the parents' house it goes from limp/wavy to ringlet-type curls from shot to shot. Also, she is wearing a lot of eye make-up in the car on the way from the airport, but once she gets to the parents' house it seems like she hardly has any make-up on.
Continuity mistake: When Robert De Niro's character is showing off the surveillance equipment inside the bear and in the room, he turns the TV on and it stays on. Later when the scene is about completed, the TV is on in one frame, and then with no-one motioning to turn it off, or having their hands near it, the TV turns off by itself. One shot it's on, the next it's not.
Continuity mistake: After the urn scene Greg and Pam are kissing and depending on where the camera is her hands are either on his cheek or more near his ears.
Continuity mistake: In the first scene where Ben Stiller is holding the (substitute) cat the camera turns away from him for a second and it's in a completely different position.
Continuity mistake: Greg's hair style and colour changes during the dancing scene at the restaurant. It looks dyed black and spiked for this scene. It switches back to normal when he is on the pay phone. Switches to spiked black when they confront him in the house. Switches back when he receives his bag outside and switches yet again when he arrives at the airport. The same is noticed early in the movie. Spikey black hair in the airport trying to check his bag and when they arrive at the house it's normal again.
Factual error: Greg's flight back to Chicago was leaving LaGuardia at 2:35am, only problem is that LaGuardia doesn't fly past midnight, they shut down. They should've used JFK which is 24/7 and only 15mins away from LGA, or Macauthur which is even closer to Oyster Bay.
Continuity mistake: During the race home scene, just after noticing Jack next to their car, Larry says "Oh look, there's Jack", and his face turns back forwards towards the front of the car, but then in a split second the shot changes and Larry is looking straight to his left (when saying "Hey Jackie").
Continuity mistake: When Greg and Pam arrive at Pam's parents' house, Greg opens the driver's door and the window is up. After Pam throws his pack of cigarettes on to the roof, he closes the driver's door and the window is now down.
Continuity mistake: When Denny sneaks in his window, he has on a jacket. Later on when they go downstairs, Pam gets the same jacket out of the closet for Greg to wear.
Continuity mistake: When Jack says, "You spray painted his tail to make him look like Jinxy didn't you?" Look closely at Bob behind him, his mouth is closed but in the next shot of Jack holding Jinxy, Bob's mouth is open.
Audio problem: When Jack talks about Kevin's ring, Pam stops drinking and coughs slightly but her mouth doesn't move.
Continuity mistake: When Jack is talking on the phone and Greg is on the roof, Jack is not wearing any jacket. Then just as he steps into the house you see his back and he's wearing the tan jacket.
Continuity mistake: When Greg borrows Jack's P.J.'s, sometimes there are initials on them others there aren't.
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.