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 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: 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: 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.
Audio problem: When Susan is playing the guitar, it has an electric guitar sound. Susan is not playing an electric guitar and neither is Hecky so the guitar should not sound like that.
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.
Revealing mistake: When the "twins" are singing "Let's Get Together" they are standing side by side. Watch closely - because of the split screen, one twin's arm disappears for just a second when she swings it upwards. It actually happens twice - middle of song and end.
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.
Revealing mistake: Susan's camp friend with the light blond hair continually repeats the last word or line of the person before her, or mouths it to be ready for her own lines. In the dining room when the twins first see each other, Susan says something like "Whatever can I do about it?" and while she's saying that, the other girl is mouthing the words, then says her own line. She also does it in the shower scene, where they're talking about revenge, the other friend of Susan's says, "Give it to her good!" then this mimicker says, "Good. The old coventry treatment..." It's pretty funny, and shows what child actors had to learn.
Other mistake: After the camping trip, in the kitchen, Maggie McKendrick remarks that the camping trip ended early. (It did so because of the blowout with Vicki.) Later, Maggie tells Mitch that she had made extra food for him and Susan for the next evening (done before they got home), because she and Sharon would be leaving in the morning. This is inconsistent.