Complex systems contain changing mixtures of failures latent within them

The complexity of these systems makes it impossible for them to run without multiple flaws being present. Because these are individually insufficient to cause failure they are regarded as minor factors during operations. Eradication of all latent failures is limited primarily by economic cost but also because it is difficult before the fact to see how such failures might contribute to an accident. The failures change constantly because of changing technology, work organization, and efforts to eradicate failures.

Source: How Complex Systems Fail, Richard I. Cook, MD, Cognitive Technologies Laboratory, University of Chicago, 1998