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.
Revealing mistake: When Hallie is about to pierce Annie's ear, Annie puts the apple underneath her earlobe and Hallie puts the needle in - you can clearly see that Annie's ear already has a hole in it.
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.
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.
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 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 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.
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.
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.
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 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.
Continuity mistake: When Annie and Hallie are playing poker, Annie puts down a pound. In the very next shot, when Hallie puts down a £5 note, the pound is gone. There wasn't a big enough pile of stuff yet to hide it.
Factual error: Copyright stamp at the end of credits reads MCMMXCVIII, that is the third M is superfluous.
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.
Audio problem: In the scene where Hallie (pretending to be Annie) meets her mom for the first time, right after her mom says "No I absolutely love it" going to the next shot, you can see Hallie taking a breath. After she closes her mouth, you hear that breath.
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.
Audio problem: When Annie is saying goodbye to her butler Martin and they perform their legendary handshake, Annie is the first to put her hand down when they stack their hands, however you hear a clapping sound.
Plot hole: The parents meet and get married on the QE II and then we jump to 11 years and nine months later, which is the time the girls get to camp. It comes out that they will both celebrate their 12th birthdays on October 11, so at the time of the camp (which must be around July/August) they are 11 years and around 9 months. So either they were born almost immediately after conception or the timeframe is off.
Revealing mistake: When Hallie and Annie are fencing with each other, Hallie forces Annie's sword into the air, and Annie runs towards the hay stack in the background to catch it. The sword lands on top of the stack of hay to the left of the screen and bounces out of sight; Annie then catches another sword that falls from above, that someone obviously dropped for her to catch.
Continuity mistake: When Annie and Hallie are dragging Meredith's airbed into the lake, a close-up of the girls at the water's edge shows the one in a blue plaid shirt with her sleeves rolled up. In a long shot behind them, her sleeves aren't rolled up.
Factual error: Placing Hallie and Annie alone in an isolation cabin without a counselor sleeping there to supervise is an enormous liability. No camp that cared about its reputation and about avoiding a lawsuit would ever put two girls in such a potentially unsafe situation.
Continuity mistake: After Annie first meets Meredith (playing Hallie), when she's sitting in the red water floater, her necklace goes from tucked inside her bathing suit to out, then to in again.