Intracardiac shunts complicating penetrating heart wounds may be unrecognized during the early postoperative period due to the insensitivity of noninvasive diagnostic tests. This case demonstrates the value of intraoperative cardiac sampling for oxygen saturation to identify an otherwise occult aorta-right ventricular fistula secondary to a precordial stab wound, ultimately progressing to a 1.9:1 shunt requiring operative repair.