swiss cheese?
I have always thought that Prof Reason's desription of aviation accidents as swiss cheese holes lining up a dangerous description, especially for training and learning purposes. It somehow implies that outcomes are naturally biased to safety, and that a long sequence of improbable events have to go pear shaped for serious events to occur.
A more helpful way to think of it is as "negative swiss cheese" - that is, that most or all critical components of a flight have been performed to a clearly safe level , for things to go " right". For significant components of the safety chain, one swiss hole may be all it takes. The other advantage of thinking about safety this way is that one day, when a really bad day comes along, the more things that routinely go "right", there is less tendency for compounding error ( ie as in AF 447) to take hold.
Also,early subtle fatigue can have a major impact on function well before self reporting occurs.