James Reason likes to explain this by invoking the metaphor of multiple slices of Swiss cheese, the cheese famous for being riddled with holes (Figure 5.3). If each slice of cheese represents a condition in the task being done, an accident can happen only if holes in all four slices of cheese are lined up just right. In well-designed systems, there can be many equipment failures, many errors, but they will not lead to an accident unless they all line up precisely.
