Tooth isolation methods used during endodontic procedures are necessary to create an aseptic field and protect the patient's alimentary tract from irrigation and instrument insult. This case details changes that can occur in mandibular cortical bone architecture after the use of a stainless steel rubber dam clamp during an endodontic procedure. A 22-year-old, otherwise healthy woman underwent nonsurgical root canal therapy on tooth #31 (mandibular right second molar, symptomatic irreversible pulpitis and periapical periodontitis.) Cone-beam computed tomographic imaging obtained between treatments revealed irregular erosive and lytic changes of the crestal-lingual cortical bone, resulting in sequestrum formation with infection and exfoliation. Continued monitoring and 6-month post-treatment CBCT imaging revealed complete resolution without further intervention. When a stainless steel rubber dam clamp is placed onto gingiva overlying mandibular alveolar bone, bony changes can occur presenting with radiographic signs of cortical erosion and may result in necrosis of the cortical bone with sequestrum formation. Knowledge of this potential outcome improves our understanding of the normal course progression after dental procedures using a rubber dam clamp for tooth isolation.