Perhaps the largest single cause of unpredicted failures in complex systems is that multiple components, supposedly independent and redundant, can all fail at the same time for unforeseeable reasons. These can be "commonmode" failures--multiple failures of identical, redundant components in the same manner--or "common-cause" failures--multiple failures, caused by a single initiating event, of components that are different from each other but aresupposed to do the same task. For example, identical valves can fail at the same time if they are all exposed to conditions for which they were not designed, or if they were designed or built wrongly: a common-mode failure

From "Brittle Power", Lovins and Lovins 1984