> Basically, the line of causation of the mishap has to pass through a metaphorical block of Swiss cheese, and a mishap only occurs if all the holes in the cheese line up.

The metaphor relies on you mixing and matching some different batches of presliced Swiss cheese. In a single block, the holes in the cheese are guaranteed to line up, because they are two-dimensional cross sections of three-dimensional gas bubbles. The odds of a hole in one slice of Swiss cheese lining up with another hole in the following slice are very similar to the odds of one step in a staircase being followed by another step.