The best treatment option for giant intracranial aneurysms (GIAs) is still debated. The authors report a case of a giant thrombosed cavernous carotid artery (CCA) aneurysm for which two sessions of flow diverter (FD) placement failed, leading to bilateral blindness. A 66-year-old man presented to an outside center with a history of rapid-onset right-sided retro-orbital pain, visual deterioration, and restricted eye movements associated with headache, vomiting, and diminished sensations on the right side of the face. He was diagnosed with an unruptured thrombosed giant CCA aneurysm and was treated twice with unsuccessful FDs. At follow-up, he developed blindness in both eyes. After 1.5 years, he presented to the authors' institution with headache, vomiting, and epistaxis, for which he underwent high-flow external carotid artery-M2 segment of the middle cerebral artery bypass grafting using the radial artery as a conduit and ligation of the internal carotid artery in the neck without any added neurological deficits. This dreadful complication of bilateral blindness after being treated with repeated unsuccessful FDs has not been reported in the literature. It could have been avoided if microsurgery had been the primary modality of treatment.