Soon after Sharon arrives at Susan's home (pretending to be Susan), Verbena says something about washing all of Susan's dirty camp clothes, but is seen putting many of them directly into the dressers in Susan's room without washing them first.
When Susan shows up in Boston and greets her Grandfather for the first time, he asks her where is the gangly, 12 year old girl she was. Earlier in the movie, she and Sharon admit they are 13, soon to be 14.
Ater the fight at the dance, a shot shows Sharon and Susan on the ground, covered in food. Susan's dress, which Sharon had cut the back out with scissors earlier, is brand new, as if it had never been cut.
At the isolation cabin, just after the rainfall, Sharon and Susan go outside and the cabin exterior and grounds are dry. They then go back inside and figure out they are twins. Sharon momentarily goes back outside and the cabin exterior is wet, as expected after a strong rainfall.
When Mitch is introduced to the minister, he is told it is "Dr. Mosby" A short time later, Mitch introduces the minister to Maggie as "Rev. Mosby" . Maggie then calls the minister "Dr. Mosby" without ever hearing him referred to by that title.
When Vicky and Edna exit Mitch's house, they get into a 1959 Lincoln 4-door. However when they are driving away, (interior shots of) the car in which they are riding is a 1959 Mercury. This is evident by the distinctive types of rear roof pillars that the cars have.
After Mitch falls into the lake, he talks with one of the twins (Sharon) and puts his wet hand on her shoulder, getting the shoulder of her dress wet. A second later the dress is dry in the spot where he put his hand.
When the girls are camping by the lake and are offered more trout for dinner, in one scene one of the girls is almost lying on her side, the scene cuts to another angle and she's sitting in the "W" position, then the scene cuts back to the previous angle, she's lying on her side again.
Just Past 1:09, Susan reveals herself to her mother and grandmother. She speaks a frequent dialogue mistake saying, "I miss not having a mother". She should have said, "I regret not having a mother", or something else that doesn't contradict the situation.
When Sharon (really Susan) is in the Rolls Royce in Boston, you can see the buildings, like apartment houses, through the back window. In the shot from the outside of the car, the street is shown, and there are none of the buildings that are seen from inside the car. It is an open street.