Revealing mistake: When Hallie trashes Annie's cabin and the huge water balloon is about to fall on Annie's head, the camera is at a downward view and Annie clearly steps into the trail of the balloon instead of jumping away. (00:18:05)
Plot hole: The parents meet and get married on the QE II and then we jump to the girls getting to camp 11 years and nine months later. We learn their 12th birthdays are on October 11, so at the time of the camp (which must be around July/August) they are 11 years and around 9 months old. They should only be 11, given it takes 9 months to grow a baby, but instead they're nearly 12.
Continuity mistake: When Dennis Quaid sees Elizabeth and falls in the pool, its broad daylight, but when she helps him and they're sitting down, its twilight and there are a lot of lights on, but when they see his fiancée they stand up and it's broad daylight again. (01:29:40)
Continuity mistake: When Annie and Hallie get out of the water bin after their fence match, the water stain on Hallie's vest changes patterns.
Continuity mistake: When Annie has to jump in the lake "butt naked," she's standing on the dock, and at one point she has noticeably pierced ears. But Annie (the Londoner) is the one who didn't have pierced ears until after the girls discovered their twinness. (00:18:20)
Revealing mistake: When Meredith meets Nick and Elizabeth at the pool, Hallie comes up on her left and says, 'Hi, Mer.' Then Annie comes up on her right and says, 'How's it going?' Meredith shrieks as she looks from one girl to the other and you can tell that Annie is not really there, because Meredith is looking over her head rather than directly at her face, which she does with Hallie.
Continuity mistake: When Hallie arrives in San Francisco she is shown getting out of the car and the door is closed, then in the close-up shot the car door is open in the background.
Factual error: If you have ever actually seen London then you can see that the route Hallie takes to get to the house would take you all around London.
Continuity mistake: When Hallie and Annie are in the isolation cabin, you see the bucket that the water is dripping into from the leak in the roof, on a stack of books. After the wind blows all of Hallie's pictures off the wall they are sitting on the ground picking them up, Annie is seen picking up Hallie's little animal (Cuppie) right next to the bucket of water, which is now on the ground with no stack of books to be seen. (00:29:07)
Continuity mistake: When Hallie wants to get a popsicle/lemonade, her hair bobble is green, but in the next shot it is white and then changes back to green again. (00:24:30)
Continuity mistake: When everyone is saying their good-byes at camp, Hallie (as Annie) takes the passport and the small carry-on and turns to get in the limo. The shot switches so you can see the limo, and Hallie (as Annie) is holding the carry-on in the other hand.
Continuity mistake: When Annie and her friends are approaching the area where the fencing is taking place, all three girls are holding tennis rackets. The shot switches to the camp counselor and back to the girls. The African American girl is no longer holding a tennis racket. The shot goes back to the counselor and then to the girls again, where all three have a tennis racket again.
Factual error: Copyright stamp at the end of credits reads MCMMXCVIII, that is the third M is superfluous.
Continuity mistake: When Hallie and Annie get out of the water bin after their fence match, they both take off their helmets, revealing Annie's messy hair. In the next few scenes, you see the hair a little less messier than before and neatly pulled back.
Continuity mistake: When Annie got hit by the huge water balloon, she got herself drenched from head to toe. But in the next scene when both the camp leaders, as well as Hallie, were inside the bunk with her, Annie was completely dry.