Despite advances in thoracic aortic surgery, the incidence of perioperative major morbidity and mortality remains significant. Hemorrhage remains an important contributor to early complications and a predictor of adverse outcomes. We describe a case of failure to separate from cardiopulmonary bypass during aortic valve, ascending and aortic arch replacement for progressive enlargement of aortic aneurysms. This was likely due to a large posterior mediastinal hematoma which was only identified post-mortem and resulted from an unrecognized sentinel tear in the distal aortic graft anastomosis. The hematoma produced major compression of the left bronchus, pulmonary artery, and pulmonary veins. Failure to detect the aortic anastomotic tear, and the subsequent hematoma expansion, led to failed separation from CPB and made all resuscitative efforts including VA-ECMO support ineffective. Therefore, we believe that locating the site of adventitial dissection and expeditiously repairing it is a critical step in managing such complication and should be done while supportive measures are provided.