Unplugged perils, lost hazards and failed mitigations

In this paper we investigate the phenomenon of unplugged perils - safety concerns which are known to some or all parties associated with the design and operation of a system, but which nevertheless result in an accident. In a small number of cases unplugged perils represent deliberate risk-taking - operation of a system despite (or because of) known dangers. Other unplugged perils result from `lost hazards' - hazards which are known to some parties associated with a system, but fatally unknown to others. A further group of unplugged perils are `failed mitigations' - hazards which are believed by all parties to have been successfully treated, but which in fact present unacceptable risk. By systematically examining accident reports across a range of industries we draw conclusions about the extent to which the various types of unplugged perils are real issues, and discuss the impact this may have on our understanding of good safety practice.