Layer of Protection Analysis is a powerful tool for quantitative risk assessments. of applied correctly, it can provide quick? and efficient guidance on what additional safeguards are needed. if any, to protect against a given scenario. If misapplied, an overly conservative calculation, of risk? may result in over-instrumentation additional life-cycle costs, and spurious trips. A nonconservative calculation of risk? could results in an under-protected system. and unacceptable risk of an undesired consequence occurring. This article describes several categories of common errors, some overly conservative and some nonconservative. Case studies of actual plant scenarios are used to illustrate. (C) 2009 American Institute of Chemical Engineers Process Saf Prog 28: 300-307, 2009