As Nancy, Bonnie and Rachelle are approaching Sarah, while she is watching Chris at football practice, Nancy is holding a book. The first time you see the book, Nancy is holding it by the spine, the second time you see it, she is holding it by the open part.
Perhaps I am misunderstanding what you mean in the scene you described, but it is not possible that she changed her grip on the book? It would only take a second to do that.
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