An interesting question, what in software causes catastrophic issues when it's broke, but also designed as it should, like the Jesus nut is for a helicopter? O

An interesting question, what in software causes catastrophic issues when it's broke, but also designed as it should, like the Jesus nut is for a helicopter? Obviously many systems have a single point of failure, and can result in lost productivity, but is that designed as it should, or cause catastrophic results like a helicopter crash?