We present the case of a 68-year-old woman, referred to our department for critical upper limb ischemia, which had occurred a few days after homolateral surgical ligamentotomy for carpal tunnel syndrome, diagnosed and confirmed by electromyography, and performed with a brachial tourniquet. The patient was later admitted for subsequent progressive necrosis of the first three fingers of the left hand, accompanied by signs of upper limb ischemia. An accessory cervical rib was identified, completely obliterating the subclavian artery distally at the origin of the suprascapular artery. A complete humeral artery occlusion was also found at the middle third of the humerus. The accessory rib was resected and the subclavian artery recanalized. A few days later, necrosis of the distal third of the first two fingers appeared and surgical resection was performed. Despite this chronic condition, the acute occlusion of collateral circles was probably induced by the brachial tourniquet. This represents a rare event, never previously reported in the literature: a case of critical upper limb ischemia due to a brachial tourniquet in a patient with misdiagnosed thoracic outlet syndrome. Until specific electrophysiological criteria for this syndrome can be found, attention should focus on history and clinical examination in patients with suspected carpal tunnel syndrome.