This introduction to redundancy includes wisdom from Warren Buffett, Charlie Munger, Confucius, Norman Fuqua and others."The reliability that matters is not the simple reliability of one component of a system, but the final reliability of the total control system"
We learn from Engineering that critical systems often require back up systems to guarantee a certain level of performance and minimize downtime. These systems are resilient to adverse conditions and if one fails there is spare capacity or a backup system.
A simple example where you want to factor in a large margin of safety is a bridge. David Dodd, longtime colleague of Benjamin Graham, observed "You build a bridge that 30,000-pound trucks can go across and then you drive 10,000-pound trucks across it. That is the way I like to go across bridges.
Looking at failure, we can see many insights into redundancy. There are many cases of failures where the presence of redundant systems would have averted catastrophe. On the other hand, there are cases of failure where the presence of redundancy caused failure. How can redundancy cause failure?