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.
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.