This paper describes the management of a failed mandibular third molar extraction, resulting in tooth displacement in the sublingual space, the discussion of the diagnosis, surgery and medico-legal considerations. A 28-year-old male patient underwent an unsuccessful attempt of the 4.8 tooth extraction. The clinician lost visual contact after luxation and the patient was not recalled for post-operative follow-up. After 24 hours, a severe trismus started. Ortopantomography and cone beam computer tomography revealed the displacement in the sublingual space. The tooth was removed under general anaesthesia with intraoral approach. The follow-up was uneventful and the paraesthetic area on the tongue did not enlarge after the retrieval. The displaced mandibular third molar is a rare but potentially serious complication of extraction. This event should be avoided with correct diagnosis and surgical technique. Cone beam computed tomography was useful to determine the three-dimensional position of the displaced tooth.