Wrong-site procedures result in significant patient harm, and prior studies have shown that—contrary to traditional assumptions—many of these errors occur outside the operating room. This analysis of 14 cases of wrong-site thoracenteses, a procedure to remove fluid from around the lung, identified several common themes in these errors. The majority of errors resulted in serious patient injury. Root cause analysis of the errors found that clinicians often failed to perform a time out and did not correctly document laterality in consent forms and clinical records. A case of a wrong-side thoracentesis that resulted in the death of a patient is discussed in a previous AHRQ WebM&M commentary.