The presence of holes in any one 'slice' does not normally cause a bad outcome. Usually, this can happen only when the holes in many layers momentarily line up to permit a trajectory of accident opportunity - bringing hazards into damaging contact with the victims. No two accidents have exactly the same positioning of the holes. Nearly all adverse events involve a combination of these two sets of factors. Active failures are unsafe acts committed by people who are in direct contact with the patient or system.
Active failures have a direct and usually short-lived impact on the integrity of the defenses.
ALL such acts, he says, have a causal history that extends back in time and up through the levels of the system. These latent conditions arise from decisions made by designers, builders, procedure writers, and top-level management. Safety involves a lot of tradeoffs. Still, he continues, much can be done to analyze the holes and eliminating them. Errors must be re-enacted immediately after an error occurs, whether fatal or not.