Continuity mistake: When the twins are in the cabin and Sharon offers Susan a Fig Newton, the cookies are is a jumble in the box. A moment later, the cookies are in a clean row.
Continuity mistake: In the cabin when they are coming up with the idea to swap places, Susan has on white sneakers in the beginning of the scene when she is on the bed, and Sharon has on the black Mary Janes. Later in the scene, with no time to change shoes, Susan is seen walking around the cabin wearing the black Mary Janes.
Continuity mistake: In the camp cabin, the two girls are talking. It cuts away from one girl, and she has an ice cream cone in her hand, then cuts back and it's gone. This keeps happening.
Continuity mistake: When the girls in Sharon's cabin have been booby-trapped, they all wake up screaming to the sound of the bugle. There are shots of them in their beds and then one shot of a girl in the middle of the cabin getting a bucket of water dumped on her head. Considering that the cabin is covered in strings and stuff how could she have gotten there in just seconds?
Continuity mistake: Mitch drives "Susan" home from the airport. After he touches her face, he puts his hand down next to him, but when it cuts, his hand is on the steering wheel.
Continuity mistake: Watch the yellow float that falls into the lake with Mitch, when Mitch is straining to see Maggie. First you can barely see it floating towards the right of the lake. Next angle, and it's behind him to the left.
Continuity mistake: When driving home to Boston from camp, they would show her in the car and the street behind her was like a big city street, then when they show the car from the outside, the street is like a small neighborhood street with totally different buildings and totally different cars (even different colors) right behind her.