Errors in medical practice may seriously harm patients. This damage is not always attributable to the action of a doctor. We must bear in mind that all human activity is susceptible of errors and that the effects of those errors will depend on the target of such activity. Therefore the consequences of doctor's errors will necessarily be the patient's health and the only possible preventive action of professionals is to take the measures to reduce the occurrence of errors to a minimum. This essay is a reflection about the nature of errors in medicine, about when they can be attributed to the action of doctors and from where they arise. We therefore analyze errors in different human activities. We distinguish errors in science, arts and techniques. We also consider errors that arise from the act itself and analyze if this is the source of errors imputable to doctors.