The authors present the case of a patient with multiple medical problems who suffers complications related to decisions made by her providers during hospitalization. The case is used as background for exploration of the concept of errors, defining preventable ones, and the mechanisms by which organizations review such errors in an attempt to uncover systems solutions. Specifically, the authors discuss the reliability of diagnostic testing, the concept of hindsight bias, and the limitation of using resources such as root cause analysis in situations when causality is difficult to define. This article is part of "Quality Grand Rounds," a series of articles published in the Annals of Internal Medicine that explores a range of quality issues and medical errors.