Patient harm resulting from errors in the diagnostic testing process is common in outpatient care, but many potentially harmful errors become "near misses" due to mitigating actions by clinicians, office staff, or patients themselves. This study analyzed voluntary incident reports from a companion study of testing errors in family medicine clinics, with the goal of identifying factors that prevented patient harm from testing errors. Errors were more likely to be mitigated if they could be more easily detected by office staff—for example, if a test was ordered incorrectly—and mitigated events resulted in less harm to patients. However, as noted in prior research, problems were noted at each stage of the testing process, implying that ambulatory clinics require comprehensive systems for ordering and following up on tests to ensure patient safety and optimal care.