The rape scene was originally supposed to take place in the woods. However, shortly before filming, Kristen Stewart discovered she had an allergy to the grass that caused her to rash, so the scene was moved to a car. When she's walking home from the party, there are leaves on her back because that scene was filmed before she learned of the allergy.
A downpour during the filming of some outdoor party scenes required the on-the-spot alteration of a few key sequences, including the replacement of some underage extras by miscellaneous crew members. Also, the indoor scene in which Andy Evans Eric Lively towels Melinda's (Kristen Stewart's) hair dry was a last minute improvisation due to the unexpected rain.