We argue that the underdeveloped concept, unintended consequences, first enunciated by Robert Merton 60 years ago, can be usefully applied to understanding the impact of mass action on the ongoing process of social structuration. We offer three apparently distinct case studies: the civil rights movement at City University of New York in the 1960s, the migration of unpapered immigrants from El Salvador beginning in the 1970s, and the unionization of the automobile industry in the 1930s. In each case, the social impact extended far beyond the most ambitious hopes of participants and included changes that were both unanticipated and unintended. In many cases the consequences were also undesired. In all cases the initial impact became embedded in social institutions that became the agency and/or object of further social change, long after the initial impetus of mass action became invisible. We offer an analytic description of the processes by which mass action affects social structure.