Background Hypochlorite solution is widely used in dental practice during root canal treatment. Although it is generally regarded as being very safe, potentially severe complications can occur when it comes into contact with soft tissue especially due to its cytotoxic features. Objective The aim of our paper is to present a case of damage of oral mucosa because of leakage of 3% hypochlorite through rubber dam during endodontic treatment. Material and methods We present a 31 years old female with necrosis of buccal mucosa during the endodontic treatment of 46. Results Three days after the procedure the patient was referred to our department for consultation and treatment. Antiseptic lavage was performed and oral antibiotic was administrated. After 5 days intraoral examination showed signs of almost full recovery. Conclusion The need for proper tooth isolation during restorative procedures is obvious. Anything that obscures the operative field negatively impacts operator efficiency and effectiveness. Visibility, patient/operator safety, infection control and the physical properties of dental materials are all compromised when proper isolation is lacking.