Errors prevention and patient safety in transfusion medicine are a serious concern. Errors can occur at any step in transfusion and evaluation of their root causes can be helpful for preventive measures. Root cause analysis as a structured and systematic approach can be used for identification of underlying causes of adverse events. To specify system vulnerabilities and illustrate the potential of such an approach, we describe the root cause analysis of a case of transfusion error in emergency ward that could have been fatal. After reporting of the mentioned event, through reviewing records and interviews with the responsible personnel, the details of the incident were elaborated. Then, an expert panel meeting was held to define event timeline and the care and service delivery problems and discuss their underlying causes, safeguards and preventive measures. Root cause analysis of the mentioned event demonstrated that certain defects of the system and the ensuing errors were main causes of the event. It also points out systematic corrective actions. It can be concluded that health care organizations should endeavor to provide opportunities to discuss errors and adverse events and introduce preventive measures to find areas where resources need to be allocated to improve patient safety.