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)
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.
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.
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.