Here’s an article from Wikipedia for anyone interested in preventing such accident such as the BP oil spill:

The Swiss Cheese Model

Models of accident causation are used for the risk analysis and risk management of human systems. Since the 1990s they have gained widespread acceptance and use in healthcare, in the aviation safety industry, and in emergency service organizations. Many of them focus on so-called the cumulative act effects.

James Reason hypothesizes that most accidents can be traced to one or more of four levels of failure: Organizational influences, unsafe supervision, preconditions for unsafe acts, and the unsafe acts themselves. In this model, an organization’s defences against failure are modelled as a series of barriers, with individual weaknesses in individual parts of the system, and are continually varying in size and position. The system as a whole produces failures when all individual barrier weaknesses align, permitting “a trajectory of accident opportunity”, so that a hazard passes through all of the holes in all of the defenses, leading to a failure.[1][2] The model includes, in the causal sequence of human failures that leads to an accident or an error, both active failures and latent failures. The former concept of active failures encompasses the unsafe acts that can be directly linked to an accident, such as (in the case of aircraft accidents) pilot errors. The latter concept of latent failures is particularly useful in the process of aircraft accident investigation, since it encourages the study of contributory factors in the system that may have lain dormant for a long time (days, weeks, or months) until they finally contributed to the accident. Latent failures span the first three levels of failure in Reason’s model. Preconditions for unsafe acts include fatigued air crew or improper communications practices. Unsafe supervision encompasses such things as, for example, two inexperienced pilots being paired together and sent on a flight into known adverse weather at night. Organizational influences encompass such things as reduction in expenditure on pilot training in times of financial austerity.[2][3]

The same analyses and models apply in the field of healthcare, and many researchers have provided descriptive summaries, anecdotes, and analyses of Reason’s work in the field. For example, a latent failure could be the similar packaging of two different prescription drugs that are then stored close to each other in a pharmacy. Such a failure would be a contributory factor in the administration of the wrong drug to a patient. Such research has led to the realization that medical error can be the result of “system flaws, not character flaws”, and that individual greed, ignorance, malice, or laziness are not the only causes of error.[4]

What happened in the Gulf was the cumulative effect of organizational influences, unsafe supervision, preconditions for unsafe acts, and the unsafe acts all lined up, i.e. all the holes in a piece of Swiss cheese were in perfect alignment.