This paper is concerned with events that were the subject of an inquest (The report of the Manitoba pediatric cardiac surgery inquest: an inquiry into twelve deaths at the Winnipeg Health Sciences Centre in 1994, Provincial Court of Manitoba, Winnipeg) into the deaths of 12 children who died while undergoing or shortly after having undergone cardiac surgery at the Winnipeg Health Sciences Centre, Man., Canada, during 1994. A notable finding of this inquest was that nurses involved with the pediatric cardiac surgery program were concerned about the competence of the surgeon and made sustained efforts throughout 1994 to have these concerns addressed. That the nurses' concerns were not taken seriously is the central problem of this paper. The work of Foucault is used to set out a ground for thinking about this problem. In this case, knowledge practices, specifically those concerning who can claim status as a credible knower, produced limits for nurses. Such limits are neither good nor bad in themselves, but rather have effects with which we must be concerned. In this case, the limits produced by certain knowledge practices had the effect of rendering the nurses' concerns irrelevant and this is significant in itself and also because it was an important part of how patient care was allowed to be compromised.