"Normal Accident Theory:
When we try to understand the causes of technological accidents, it often turns out to be very difficult to pinpoint exactly what went wrong. The reason for this is that technologies are intrinsically complex and depend on many things working closely together: Materials and components of different quality are structured into tightly engineered sub-systems, which are operated by error-prone humans in not always optimal organisational structures, which in turn are subject to production pressures and all kinds of managerial manoeuvring.
Failure in just one part (material, sub-system, human, or organisation) may coincide with the failure of an entirely different part, revealing hidden connections, neutralised redundancies, bypassed firewalls, and random occurrences for which no engineer or manager could reasonably plan.
This is what 'Normal Accident Theory' is about: When a technology has become sufficiently complex and tightly coupled, accidents are inevitable and therefore in a sense 'normal'.
Accidents such as Three Mile Island and a number of others, all began with a mechanical or other technical mishap and then spun out of control through a series of technical cause-effect chains because the operators involved could not stop the cascade or unwittingly did things that made it worse. Apparently trivial errors suddenly cascade through the system in unpredictable ways and cause disastrous results."