Introduction: There are currently no established guidelines to determine which cases general practitioners should refer to an endodontist for root canal treatment. The American Association of Endodontists (AAE) has developed the EndoCase mobile application (ECA), which utilizes either a full or abridged rubric to assign case difficulty level and provide referral guidelines to general practitioners and dental students. Objective: The objective of this study was to determine whether the abridged criteria of the EndoCase application can help predict the incidence of procedural errors in nonsurgical root canal treatment of mandibular molars in an undergraduate dental clinic based on the difficulty level. Methods: A list of patients who received primary root canal treatment on mandibular first molars in the undergraduate dental clinic from 2015-2020 was obtained. Ninety patients qualified for inclusion. Case difficulty level was assessed using the ECA by three providers with differing levels of experience. Incidence of procedural errors was determined from post-operative radiographs by two calibrated independent observers. Results: The most common endodontic mishaps were errors during access cavity preparation followed by the presence of voids in the root filling, with an incidence of 54.4% and 45.6%, respectively. There were no significant differences regarding case difficulty level and the incidence of total procedural errors nor number of treatment visits. Of the individual error types, the presence of obturation >2mm short of the radiographic apex was weakly correlated with case difficulty level (r = 0.226, pConclusion: There is minimal correlation between the difficulty level of mandibular molars determined by the ECA and the number of treatment visits or overall incidence of procedural errors.