Marianne's Reckless Behavior
1. Allowing Willoughby to cut a lock of her hair. This action had Mrs. Dashwood and Elinor Dashwood thinking that Willoughby and Marianne had a tacit agreement. They therefore went along with her forward behavior, shocked to find out later that Willoughby had made no long-term promises.
2. Walking recklessly in the rain to see Willoughby's house, Combe Magna, and almost catching her 'death' of cold. This scene was not in Jane's book, but both recent movie adaptations featured this change, which led to the dramatic sick scene.