In recent decades, an interdisciplinary quality assurance (QA) movement has emerged in health care studies, which has included increased attention to medical errors. Implicit in this QA effort is a conflict between (1) external agents encouraging the medical profession to adopt strategies for reducing errors and (2) sociological characteristics of medical practice that systematically inhibit the uptake of these strategies. Using interviews with providers and observations in two diabetes clinics in a large Midwestern city in the USA, we examine how providers understand error in their work, as well as how they think about failures in care and efforts to standardize and impose guidelines in care. We find that the prototypical vocabularies of medical error and QA, which have been largely oriented to acute illness care, are systematically mismatched to ambiguities introduced by chronic illness. These ambiguities create problems for the definition of medical errors, the collection of relevant information, the determination of long-term treatment goals, and the application of standardization efforts. Considered together, these mismatches imply diminishing returns for health policy efforts focused on reducing medical error as part of a larger QA agenda.