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: 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: 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.
Other mistake: When the girls leave camp, if you look closely it is obvious that one of the girls (the one in the distance) is a double who does not look like Susan / Sharon at all.
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.
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.
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: When Susan goes outside to sit on steps after the rain has stopped you can see her double siting on the bunk looking at her photo album.
Revealing mistake: When they are driving home from the airport, you can see the reflection of the green screen on the car hood.