How do you know you are in an error chain? How do you differentiate a normal step in a sequence from one sufficiently abnormal to, which uncorrected, may lead to the incident or accident? An error chain in retrospect seems easy to define, but with cool logic you could say ... “We cannot respond to something that we do not recognize...” The Swiss cheese model can be used to demonstrate when the holes line up to lead to an accident. But if we move one of the slices just a little, new possibilities appear. That is the situation faced by the maintenance engineer. The possibilities are endless.