Cardiac tamponade is a common cause of hypotension and low output after cardiac surgery. The authors present a case of false tamponade diagnosis on transesophageal echocardiography in a patient who underwent mitral valve replacement and coronary artery bypass grafting. The diagnosis was not confirmed at surgery, and the cause of the abnormal right atrial filling was a prominent Eustachian valve trapped in the suture for correction of an iatrogenic inferior vena cava laceration. Such a complication must be kept in mind when repairing inferior vena cava lacerations and should be more widely known.