This case describes a 45-year-old woman with significant respiratory distress secondary to a left-sided pleural effusion that mandated an urgent thoracentesis. An adverse event occurred when the physician performed the procedure on the incorrect side of the patient. Results of the incident investigation followed by a discussion of medical errors models, common errors types, human factors considerations, and conditions that contribute to error are presented. Pertinent case-specific and general concepts of a system approach to reduce this type of medical error are discussed, and educational recommendations are offered.