“Very few unsafe acts will result in damage or injury. In a highly protected system, the probability that the consequences of an isolated action will penetrate the various layers of defense is vanishingly small. Several causal factors are required to create a ‘trajectory of opportunity’ through these multiple defenses. Many of the causal contributions will come from latent failures in the organizational structure, or in the defenses themselves. Others will be local triggering factors. These could be a set of unsafe acts committed during some atypical (but not necessarily abnormal) system state. Examples of the latter are the unusually low temperature on the night preceding the Challenger launch, the voltage-generator tests carried out just before the annual maintenance shut-down in the Chernobyl-4 reactor, and the nose-down trim of the Herald of Free Enterprise because of a combination of unusually high tide and unsuitable docking facilities.”
The contribution of latent human failures to the breakdown of complex systems, 1990, J.Reason
