Although vertical root fracture (VRF) is mostly found in endodontically treated teeth, it also occurs spontaneously. If VRF is recognized after endodontic treatment, it is considered to be iatrogenic and can lead to legal trouble. However, legal problems can be averted if the dentist can prove that the VRF existed before endodontic treatment. This case report describes an unusual, spontaneous VRF in an endodontically treated tooth and presents a useful tip for determining whether a fracture is iatrogenic. We performed nonsurgical endodontic treatment on a mandibular first molar with irreversible pulpitis. After 6 months, the patient revisited with localized swelling, and we diagnosed VRF of the mesial root. We extracted the tooth and prepared it for microscopic examination. We found gutta-percha in the fracture line of the transversely sectioned root, and it appeared to have penetrated to the fracture line through the force generated from the filling. The patient was informed and agreed that the fracture occurred spontaneously before treatment. This case demonstrates the time point of VRF occurrence by identifying the presence of gutta-percha in the fracture line. We suggest that this procedure can be used to demonstrate whether VRFs in endodontically treated teeth are spontaneous or iatrogenic.