In safety-critical situations, multiple layers of defence are designed into the system to prevent an initial event progressing to an accident. These defences may include processes (e.g. regular inspections), technology hardware (e.g. provision of redundant systems), and functionality (e.g. fail-safe design), among others. Each of these protects against some events, but not all. Also, these defences are of variable efficacy at any particular time, e.g. due to the variability of human attentiveness. Consequently coincidences may arise whereby all the defences fail, and the initial event proceeds to an accident.