The most frequent mistake during access cavity preparation in mandibular incisors is lingual perforation due to the thin lingual cortical plate and the angulation of the canal system. Operators often misjudge the position, leading to overextension of the access cavity lingually. This can compromise tooth structure and complicate treatment. Proper knowledge of the root canal anatomy and conservative preparation limited to the pulp chamber reduces the risk of lingual perforation. Maintaining access within the anatomical confines is essential for successful endodontic therapy in mandibular incisors.

Reference: Cohen's Pathways of the Pulp, 12th Edition.