This paper describes10 types of flaws that have occurred in social experiments.  Each flaw is illustrated with examples from previous experiments.  Some of these problems result in minor hurdles, while others cause experiments to fail—that is, the experiment is unable to provide a valid test of the hypothesis of interest.  An accompanying summary table lists the flaws, indicates the circumstances under which they occur, their potential seriousness, and approaches for minimizing them.  The most important of the flaws are response bias resulting from attrition; a failure to adequately implement the treatment as designed; and too small a sample to detect impacts.