This paper describes a detailed ethnographic study of the design problems of a major national IT system in the UK- The Integrated Children’s System (ICS). The implementation of the ICS has disrupted social work practice and engendered growing professional resistance, prompting a fundamental review of its design. Marshall McLuhan’s concept of chiasmus is a central feature of our analysis of the vicissitudes of ICS. Chiasmus refers to the tendency of any system, when pushed too far, to produce unintended contradictory effects, and is an intrinsic feature of the behaviour of complex, socio-technical systems. The dysfunctions of the ICS provide a pertinent, large-scale example. The ICS constitutes an attempt, via technological means, to re-organize child welfare services in the UK. Whilst aimed at improving child safety, the ICS has had the opposite effect of increasing the potential for error. This chiasmus has been exposed through the multi-site ethnography reported here, which shows how rigidly designed processes, enforced by IT systems, force social work professionals into unsafe investigative and recording practices which increase the risk of errors. The paper ends by proposing an alternative approach to design, based on socio-technical precepts, emphasizing the principles of minimum critical specification, user-centeredness and local autonomy.