Common Mode Failure

Common mode failure is a failure that simultaneously affects multiple redundant components, defeating the architectural assumption that failures are independent and can be absorbed by backup systems. Unlike independent component failures, which redundancy is designed to survive, common mode failures strike at the correlation structure of the system itself. They are the enemy of safety engineering because they transform reliable architectures into single points of failure without anyone noticing.

The classic example is the diversity defense problem: two identical backup systems may fail together because they share a design flaw, a manufacturing defect, or a common environmental trigger. The Therac-25 radiation therapy accidents involved a race condition that could occur in both the primary and backup safety interlocks because they shared software. Common mode failures teach that redundancy without diversity is not safety — it is theatrical safety, a performance of risk management that conceals correlated vulnerability.