Quoted: That's why I'm skeptical of people who look at some catastrophic failure of a complex system and say, "Wow, the odds of this happening are astronomical. Five different safety systems had to fail simultaneously!" What they don't realize is that one or two of those systems are failing all the time, and it's up to the other three systems to prevent the failure from turning into a disaster. You never see a news story that says "A gas refinery did not explode today because simultaneous failures in the first, second, fourth, and fifth safety systems did not lead to a disaster thanks to a correctly-functioning third system." The role of the failure and the savior may change over time, until eventually all of the systems choose to have a bad day all on the same day, and something goes boom.