Recent discussions on improving health outcomes in the hospital setting have emphasized the importance of classification of mistakes in health care institutions These discussions indicate that the existence of a shared classificatory scheme among members of the health team indicates that errors in patient care are recognised as significant events that require systematic action as opposed to defensive, one-dimensional behaviours within the health institution. In Nigeria, discussions of errors in patient care are rare in the literature. Discussions of the classification of errors in patient care are even rarer. This study represents a first attempt to deal with this significant problem and examines whether and how mistakes in patient care are classified across five professional health groups in one of Nigeria’s largest tertiary health care institutions. The study shows that there are wide variations within and between professional health groups in the classification of errors in patient care. The implications of the absence of a classificatory scheme for errors in patient care for service improvement and organisational learning in the hospital environment are discussed.