Process hazard analysis (PHA) has become the standard way of identifying possible accidents in processes, However, despite the large number of studies that have been performed since OSHA's Process Safety Management standard was issued in 1992, accidents still occur. When this happens the effectiveness of PHA and the quality of the studies performed may be questioned. Litigation can arise, and attorneys may try to make the case that someone was negligent because the PHA did not identify the accident that occurred. While it is possible that PHAs may miss accident scenarios owing to the quality of the study performed, there are other reasons why scenarios may not be identified. This paper identifies and describes such reasons.