Proper hazard identification in safety reports has become progressively more difficult to achieve. Several major incidents in Europe in recent years, such as Buncefield and Toulouse, were not even considered by their site Seveso II Safety Case. One of the reasons is that available hazard identification (HAZID) methodologies take no notice of least likely or unknown events. Non identified scenarios thus constitute a latent risk, whose management is extremely complex and open-ended. Hence this work aims to investigate the issue of atypical scenarios and explain how they could have been identified. An in-depth historical analysis of some of these incidents has been performed, in order to outline general features of plants in which they occurred, their causes, consequences and lessons learned. This analysis has followed a precise common scheme, which allowed a systematic approach to the problem. Based on the findings, failures connected to risk management and risk appraisal have been identified. Checklist questions and recommendations will be compiled, making efforts to improve the general safety of industry. Attempts will be made to review well-known HAZID methodologies, and thus the process of risk appraisal, with the purpose of improving the HAZID processes to identify atypical scenarios.