When Marcus and Rachel are sitting on the bench in the park, the woman sitting on the bench next to them can be seen reading the book, "Something Blue" by Emily Giffin - "Something Blue" is the sequel to the book "Something Borrowed".
Near the end, Darcy goes to Rachel's apartment. She has nothing with her. When we see her storm outside the building, she grabs Marcus' pizza wrapper and walks away. Then the shot switches to her from the back and she has a brown purse, no wrapper. The purse was sitting beside Marcus on the stoop but we don't see her pick it up.
When everyone is playing badminton in the Hamptons, right before they start playing for secrets, Ethan opens an Heineken bottle. Few seconds later, he's opening it again.
In the last shot with Darcy and Rachel, Darcy's hair in front is down by her face. Darcy walks by Rachel and when she turns around, her hair is partially tucked behind her ear. No indication is made that she does this.