Spontaneous thrombosis of a giant intracranial aneurysm with parent artery occlusion is known. The exact mechanism is however unclear and various theories have been proposed. We present an unusual case of an angiographically documented cervical internal carotid artery (ICA) dissection, which led to total occlusion of the ICA distal to the dissected site, with acute cessation of forward blood flow. This resulted in acute upstream thrombosis of the giant cavernous carotid artery aneurysm and an acute cavernous sinus syndrome-like presentation.