When a patient with multiple, complicated conditions is admitted to a hospital and risky procedures are performed that result in adverse outcomes, the difficulties inherent in determining whether and when a preventable medical error has occurred must be addressed. This article analyzes the case of a 40-year-old woman with a history of chronic aortic dissection and pericardial effusion who was admitted to a teaching hospital with unilateral swelling of her left breast and arm accompanied by dyspnea. During her hospitalization, the patient developed multiple complications from the diagnostic and therapeutic procedures that were performed. The authors argue that this case illustrates some limitations of routinely undertaking time-consuming and costly reviews, or "root-cause analyses," as a patient safety strategy when they are unlikely to reveal remediable "errors" or to suggest better systems of care that will prevent errors. The ability to establish causality through post hoc reviews is the linchpin in the recommendation for widespread adoption of error reporting and reviews. When causality is not established, it is impossible to know whether any changes adopted as a result of the reviews will be effective. This case, in which the causal pathways to the adverse events are very uncertain, may be much more typical than the egregious errors featured in a classic root-cause analysis. The authors recommend that the relative merits of this approach to patient safety be compared with other proven, cost-effective interventions to improve quality, such as appropriate treatment of myocardial infarction or depression, before scarce resources and enormous human capital are allocated for widespread implementation.