A 26-year-old woman was referred by a dentist to an oral and maxillofacial surgeon after an unsuccesful attempt to remove a mandibular third molar. A panoramic radiograph showed the remaining root of the third molar and the patient suffered from insensitivity of the lower lip. The root of the third molar was removed surgically. The sensitivity of the lip recovered completely within three months. Injury of the inferior alveolar nerve and the lingual nerve is a serious complication after mandibular third molar removal. The inferior alveolar nerve is at risk for injury if the third molar root is intimately connected with the mandibular canal. Refraining from preoperative clinical and radiographic diagnostics is malpraxis. Surgical skill is a prerequisite to reduce the risk of nerve injury.