The anatomic anomalies of the supra-aortic trunks and their branches are comparatively frequent observations. They often remain asymptomatic throughout life although in some cases, especially if nothing is known of them, they may complicate surgical interventions involving that anatomical region. The case of a female patient who, during thyroidectomy, suffered the ligature of her external carotid artery which was taken mistakenly for the thyroid, is reported. Dissection of the area made it possible to identify an anomaly of the vascular structures with a carotid bifurcation which turned out to be very low, a very deep internal carotid artery on the prevertebral fascia and an absent lower thyroid artery. In the postoperative period, the patient was subjected to MR of the neck. This showed an anatomical anomaly of the left carotid bifurcation which was much lower than normal. An attempt was made to reconstruct the external carotid artery with a saphenous vein but unsuccessfully. The incident did not have serious complications for the patient but it might be asked whether, given the low cost and the absolute non-invasiveness of the examination, it might not be worth while to carry out a duplex scan of the supra-aortic trunks before going ahead with any operation in the cervical region, in consideration above all of the dangerous nature of these vascular structures and of the usefulness of studying pathologies that often remain silent until the occurrence of an event which might well be catastrophic.