Marcia and Jan not wearing underwear
Upset about the fact that the series wouldn't include storylines about more real-life teen issues in later seasons, Maureen McCormick revealed in her memoir that she and Eve Plumb retaliated by not wearing bras and panties while filming some episodes.
One day, producer-director Lloyd Schwartz noticed while watching dailies and confronted them about it. From that point on, he would meet them on set before filming scenes and put his hand on their backs to make sure the young girls were wearing bras.