A 72-year-old female patient with subarachnoid hemorrhage was diagnosed to have posterior communicating artery aneurysm by preoperative cerebral angiography. The patient underwent a left pterional craniotomy to approach a left posterior communicating artery aneurysm. During the procedure, an unexpected vascular anomaly was discovered. On further dissection, we confirmed a ventral aneurysm of the left coiled intracranial internal carotid artery (ICA) during the operation. However, we could not expose the neck of the aneurysm satisfactorily; hence, we finally used endovascular treatment to cure the patient. This demonstrates that the ICA may become coiled and thus make the ventral aneurysm of coiled intracranial ICA look like a posterior communicating artery aneurysm on cerebral angiography. Knowledge and anticipation of anatomic variants of the ICA are essential to avoid misdiagnosis and possible fatal hemorrhage during surgery.