We present a case of severe postoperative hypercarbia in a patient with severe COPD. Hypercarbia and respiratory acidosis continued to increase despite maximal ventilation, bronchodilator therapy, sedation, and paralysis. Mistaken use of non-partitioned ventilator circuit was the cause of the hypercarbia. The ventilator's self-test function failed to detect the error. We changed to a partitioned-lumen circuit, with much less ventilation dead space, and the hypercarbia resolved immediately.