The more holes in a system’s defenses, the more likely it is that errors will result in incidents or accidents. This only occurs in certain circumstances, when all holes ‘line up’. Usually, if an error has breached the engineering defenses, it is detected and handled when it reaches the flight operations defenses. However, occasionally in aviation, an error can breach all the defenses and a catastrophic situation ensues. It is important to understand that, in general, accidents need more lines of defense to fail.