Deliberate mistake: The lake that the girls splash about in is very shallow, as they easily stand and play in it. Roberta does a head-first dive from a high tree, into it. There's no chance she wouldn't have hurt herself. (00:32:35)
Deliberate mistake: When Roberta copies the article about her mothers death, the machine obviously copies the page too fast. It copies it immediately, yet in the 1970's copy printers were incredibly slow.
Answer: Reshooting certain scenes would most likely necessitate new stunt people, as the first one(s) would most likely be hard to get back.