No test ever invented has been as successful as the Papanicolaou smear in preventing cancer. Despite its remarkable success in cervical cancer prevention, however, the Papanicolaou smear is not a perfect test. Sampling errors account for a significant number of false-negative cases. Diagnostic errors occur in even the finest cytology laboratories. Yet, for women who develop cervical cancer, these errors pale in comparison with failure to screen patients adequately in the first place. This paper examines some of the problems that result in failure of the Papanicolaou smear to prevent cervical cancer, with an emphasis on common problems in Papanicolaou smear interpretation.