A contextual error occurs when a physician overlooks elements of a patient's environment or behavior that are essential to planning appropriate care. In contrast to biomedical errors, which are not patient-specific, contextual errors represent a failure to individualize care. To explore the frequency and circumstances under which physicians probe contextual and biomedical red flags and avoid treatment error by incorporating what they learn from these probes. An incomplete randomized block design in which unannounced, standardized patients visited 111 internal medicine attending physicians between April 2007 and April 2009 and presented variants of 4 scenarios. In all scenarios, patients presented both a contextual and a biomedical red flag. Responses to probing about flags varied in whether they revealed an underlying complicating biomedical or contextual factor (or both) that would lead to errors in management if overlooked. 14 practices, including 2 academic clinics, 2 community-based primary care networks with multiple sites, a core safety net provider, and 3 U.S. Department of Veterans Affairs facilities. Primary outcomes were the proportion of visits in which physicians probed for contextual and biomedical factors in response to hints or red flags and the proportion of visits that resulted in error-free treatment plans. Physicians probed fewer contextual red flags (51%) than biomedical red flags (63%). Probing for contextual or biomedical information in response to red flags was usually necessary but not sufficient for an error-free plan of care. Physicians provided error-free care in 73% of the uncomplicated encounters, 38% of the biomedically complicated encounters, 22% of the contextually complicated encounters, and 9% of the combined biomedically and contextually complicated encounters. Only 4 case scenarios were used. The study assessed physicians' propensity to make errors when every encounter provided an opportunity to do so and did not measure actual error rates that occur in primary care settings because of inattention to context. Inattention to contextual information, such as a patient's transportation needs, economic situation, or caretaker responsibilities, can lead to contextual error, which is not currently measured in assessments of physician performance. U.S. Department of Veterans Affairs Health Services Research and Development Service