The purpose of this case report is to show and discuss the possible causes why the endodontic treatment of an upper anterior tooth of a patient with a misfit prosthetic restoration and post failed endodontically, through scanning electron microscopy (SEM) analysis. of an apical fragment extracted in periapical surgery and by clinical and radiographic findings. The findings demonstrate that marginal filtration, poor adaptation of the endodontic filling material and, therefore, the entry of bacteria into the root canal system, were the main possible causes attributed to the failure of the endodontic treatment of this tooth, with the subsequent formation of the periapical lesion.