This report presents a case in which undiagnosed coronal leakage resulted in failure in the endodontic management of a tooth. Coronal leakage occurred during root canal treatment as a result of the presence of deficient composite resin fillings and secondary caries. Despite repeated visits of cleaning and dressing, the canal continued to be contaminated and symptoms persisted. On referral, the reason for treatment failure was diagnosed. The tooth was successfully treated by the replacement of the deficient fillings, after the elimination of underlying caries. Symptoms resolved enabling the completion of the root canal treatment.