Fortunately, most errors do not lead to accidents. Accidents often have numerous contributing causes, no single one of which is the root cause of the incident.
James Reason likes to explain this by invoking the metaphor of multiple slices of Swiss cheese, the cheese famous for being riddled with holes. 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. Any leakage-passageway through a hole-is most likely blocked at the next level. Well-designed systems are resilient against failure.
This is why the attempt to find "the" cause of an accident is usually doomed to fail. Accident investigators, the press, government officials, and the everyday citizen like to find simple explanations for the cause of an accident. "See, if the hole in slice A had been slightly higher, we would not have had the accident. So throw away slice A and replace it." Of course, the same can be said for slices B, C, and D (and in real accidents, the number of cheese slices would sometimes measure in the tens or hundreds). It is relatively easy to find some action or decision that, had it been different, would have prevented the accident. But that does not mean that this was the cause of the accident. It is only one of the many causes: all the items have to line up.
You can see this in most accidents by the "if only" statements. "If only I hadn't decided to take a shortcut, I wouldn't have had the accident." "If only it hadn't been raining, my brakes would have worked." "If only I had looked to the left, I would have seen the car sooner." Yes, all those statements are true, but none of them is "the" cause of the accident. Usually, there is no single cause. Yes, journalists and lawyers, as well as the public, like to know the cause so someone can be blamed and punished. But reputable investigating agencies know that there is not a single cause, which is why their investigations take so long. Their responsibility is to understand the system and make changes that would reduce the chance of the same sequence of events leading to a future accident.
The Swiss cheese metaphor suggests several ways to reduce accidents:
Add more slices of cheese.
Reduce the number of holes (or make the existing holes smaller).
Alert the human operators when several holes have lined up.