A traumatic carotid-cavernous sinus fistula (CCSF) was prematurely suspected following the detection of arterial-like hemoglobin oxygen saturation values, sampled from a catheter placed for cerebrovenous monitoring. A high-resolution scan of jugular foramina revealed that the catheter tip had been unintentionally placed in the inferior petrosal sinus, contralateral to the CCSF, instead of in the superior jugular bulb. Jugular bulb hemoglobin oxygen saturation (SjO2), ipsilateral to CCSF, later approached arterial hemoglobin oxygen saturation (SaO2) values. The possibility and consequences of unintentional catheterization of the inferior petrosal sinus, and of extracerebral contamination of blood in the jugular bulb due to blood in the inferior petrosal sinus, are discussed. We also discuss the reliability of SjO2monitoring in the present CCSF case.