Normal accident

Definition: 

If interactive complexity and tight coupling – system characteristics – inevitably will produce an accident, I believe we are justified in calling it a normal accident, or a system accident. The odd term normal accident is meant to signal that, given the system characteristics, multiple and unexpected interactions of failures are inevitable. This is an expression of an integral characteristic of the system, not a statement of frequency.

Definition: 

Normal accidents occur when complex machine elements malfunction, either individually or interactively, and human operators misjudge what is happening and respond with actions that accelerate the deterioration of the situation.