We all know that everyone makes mistakes – but are everyday mistakes (misplacing a household item or misdialling a telephone number) really equivalent to inadvertently administering the wrong drug to a patient? There is an understandable view that professionals are trained and paid precisely to ensure that they do in fact do the right thing. The problem of iatrogenic harm in healthcare described in chapter 2 demands a response. The first reaction to accidents in medicine is often punitive, and based on a denial of the nature of human error. The culture of clinical practice is in general one of relentless dedication to high achievement and the medical profession is the foremost culprit in perpetuating the myth of professional infallibility. It is not surprising that the courts and disciplinary authorities have taken their lead from doctors themselves and have at times seemed to treat any kind of failure in medical practice as unacceptable.

In chapter 2 we discussed some of the processes involved in human cognition and described the way in which the mind may mislead an actor and create a situation in which bizarre and apparently inexplicable actions become perfectly understandable. We now attempt to distinguish between different types of error and investigate whether it is possible to predict which type of error is most likely to occur in a given situation. We shall discuss reasons for believing that errors (in contrast to violations) are both understandable and inevitable, even for a highly trained and regulated professional.