Abstract : Root canal treatment (RCT) errors such as ledges, perforations, blockages, broken instruments or overfilling could lead to bad prognosis and treatment failure. The assessment of such errors at preclinical stages of dental students could highlight aspects that require improvement in endodontic teaching. This study analysed dental radiographs to determine the error types that could occur during RCT performed by 4th year dental students at preclinical stages. Different types of endodontic errors were investigated in root canal-treated teeth (n = 1320 extracted teeth). The treatment was previously performed with approximately consistent preparation using step-back hand instrumentation and obturation with cold lateral compaction. Two examiners assessed the radiographs following achieving intra- and inter-examiner agreements. The data were analysed statistically using SPSS software. The results showed significant differences in the occurrence of different types of errors during RCT (P≤0.05). The errors were less likely to occur during the phases of opening access and instrumentation than during the phase of obturation. In this final phase, errors such as underfilling and voids were most frequent. In conclusion, the quality of RCT was poor, especially at the phase of obturation. Improving pre-clinical teaching methods, such as increasing training sessions and the number of supervisors, seems to be mandatory to reduce such technical errors