Therapeutic failures are related not always to professional guilt. There are many cases, indeed, in which therapeutic failure as well as the damage consequent to the intervention are related not to the doctor’s guilty but to events that cannot be prevented. These events are legally defined as “complications”. On the contrary, if the medical behaviour is negligent and/or unskilled and/or imprudent, and if this kind of behaviour determines an objective damage to the patient with a causality nexus between the culpable medical conduct and the patient’s damage, the hypothesis of “professional error” can be shaped. In this last situation, the doctor is called to answer legally of own acts, for how much it concerns his/her “professional liability”. Surgical injury of the lingual nerve is an intra-operative accident which can be classified sometimes as “complication” and sometimes as “professional error”. Lingual nerve injury can occur during many surgical procedures, most of them in oral surgery, among which the lower third molar extraction is certainly the most frequent. The incidence of lingual nerve injury in the international literature is only reported regarding third molar surgery and it ranges from 0,1% to 22%. Lingual nerve damage mainly determines somatic and gustative omolateral sensitivity disturbances. Somatic disturbances involve the anterior two thirds of the half tongue, the mucosa of the half part of the floor of the mouth, and the mucosa of the internal plate of the alveolar process; gustative disturbances involve the anterior two thirds of the omolateral half of the tongue. Aim Aim of the present study is to identify the possible factors which make to consider lingual nerve injury as a “professional error” or as a “complication” and therefore which can motivate or not the imputability of the surgeon. Discussion and conclusions In the legal view, factors which can discriminate between “professional liability” and “fatality” in nerve injury occurrence should be mainly found in the two different concepts of “expectability” and “avoidability” of the nerve damage. During surgery in the posterior areas of the mandible, intra-operative injury of the lingual nerve is a widely expectable event, so that it can be considered a statistically relevant surgical “complication”; on the other side the avoidability is discriminating on the occurrence of the lingual nerve damage, in relation to surgical adopted and adoptable shrewdness’s and cautions. Lingual nerve damage is considered “not avoidable” if a serious anomalous nerve running is present or if the damage occurs during the injection for the regional anaesthesia of the lingual or/and the inferior alveolar nerves at the mandibular hole; in the latter case, the needle is inserted in the tissues following anatomic reference points that are individual and therefore extremely variable. On the contrary, the damage is considered “avoidable” if all necessary cautions and means are not used to avoid the nerve injury. These technical expedients are widely reported in the literature and they change in relation to the kind of surgery. The lack of their application delineates the requirements to ascribe the damage to a professional guilt. In this view, if the nerve damage is adequately cleared to the patient pre-operatively, if a valid “informed consent” is obtained and if all technical expedients are applied to prevent such a nerve “complication”, its occurrence answers to the aforesaid requirements of “unavoidability” and it does not represent a “professional guilt”.