"Normal" accidents, or system accidents, are so-called by Perrow because such accidents are inevitable in extremely complex systems. Given the characteristic of the system involved, multiple failures which interact with each other will occur, despite efforts to avoid them. Perrow said that operator error is a very common problem, many failures relate to organizations rather than technology, and big accidents almost always have very small beginnings. Such events appear trivial to begin with before unpredictably cascading through the system to create a large event with severe consequences.

Perrow identifies three conditions that make a system likely to be susceptible to Normal Accidents. These are:

• The system is complex
• The system is tightly coupled
• The system has catastrophic potential

[Wikipedia]