The hospital emergency department (ED) is a risky environment, often subject to litigation for negligence. Risk is defined as an avoidable increase in the probability of an adverse outcome for a patient. With the aim of identifying the sources of risk, this study carried out participant observation and collected critical incidents in two EDs in the UK for a period of 30 months. Active failures included delay in beginning investigations or treatment, failure to obtain diagnostic information, misinterpretation of diagnostic information and the administration of inappropriate treatment. Three latent conditions underlay these failures: patients' unrestricted access to the ED, cognitive errors by individual members of staff and a strict horizontal and vertical division of labour. An analysis of the incidents resulting from the third latent condition identified a contradiction between the division of labour and working conditions in the ED. The paradigm circumstances under which this contradiction can result in active failures are described. The management of risks arising in this way could be improved by developing a workplace culture in which 'sapiential authority'--authority derived from experience, special access to information or being at hand in an emergency--is recognised in addition to authority derived from a formal status. However, as long the contradictions between the division of labour and working conditions remain, accidents should be considered normal events.