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 is entering her home in London, she puts her hand on the doorknob and pushes the door open. In the next shot when it shows the entryway with the door opening, there is no doorknob.
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.
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.
Character mistake: When Chessy confronts "Annie" when she's presenting her with the big breakfast, she's talking about when Annie was on the phone the night before. Then she says her father overslept, when she actually meant to say and "and you overslept," as the conversation doesn't make any sense saying that the father overslept. (00:58:00)
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.
Factual error: At the hotel, Chessy's door to her room stays open after she goes into the other room to discuss the whole 'dad doesn't know' matter. In real hotels, the guest-room doors have auto-closing mechanisms for fire safety and security. The door would have swung closed and locked Chessy out, but even though this is a 4 or 5 star hotel which would be fully equipped as far as modernity's, it doesn't.
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. (00:14:35)
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.
Continuity mistake: When Hallie (posing as Annie) arrives in London, she is seen sitting on the left seat in the car when she's on her way home, passing by the various spots in London. However, when she reaches home, she is sitting on the right side of the car. Unless she was sitting alone, this is not logical as Martin was sitting together with her at the back of the car.
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.
Revealing mistake: Annie and Hallie come downstairs at the hotel dressed in the same outfits, as their parents tell them to hurry before their mother misses her flight back to England. Nicholas Parker leans over and looks at one of the girls in the eyes and says "This one's Hallie" and she replies, looking straight ahead at him in the eyes. When the other twin speaks, she's looking straight ahead as well, which would technically mean she is looking at nothing.