A young woman without symptoms underwent repair of an ostium secundum atrial septal defect, and exertional dyspnea developed postoperatively. This was found to be due to arterial oxygen desaturation which was exaggerated in the upright position and with exercise. Contrast echocardiography confirmed a right-to-left shunt at the atrial level that was shown only with femoral venous contrast injection and not with upper extremity venous injection. Transesophageal echocardiography and subsequent surgical exploration found that the Eustachian valve had been mistaken for the inferior rim of the defect and sutured to the upper rim of the defect. This created a channel through which blood from the inferior vena cava could be partially deferred to the left atrium.