Despite efforts to improve patient safety, medical errors (MEs) continue to recur. Proper utilization of reported MEs can be effective in preventing their recurrence. This study investigated the errors reported in 3 years and examined the factors affecting them. This descriptive analytical study was conducted using the errors reported in 20 hospitals under the auspices of one of Iran's medical universities from 2018 to 2020. All reported errors were investigated by an expert panel. In total, 6584 reported errors were grouped into four main categories based on the type of error. The highest reported errors were related to the management and treatment procedures. Analyses of the factors influencing medical errors revealed that 15 factors affected the occurrence of errors. An increasing trend of error was found in 9 of the 15 identified factors. Incorrect documenting of the physician's order in the nursing Kardex and noncompliance with the patient identification guide were the highest with 16.03 and 15.47%, respectively. The most identified factor was the incorrect registration of the physician's prescription on the nursing card; therefore, it seems that the use of computerized physician order entry should be considered. Furthermore, the mere existence and training of patient safety guides cannot help prevent errors. Not only should the underlying causes of errors be carefully identified and investigated but it also requires serious determination to follow the patient's safety instructions from the highest to the lowest levels of the health system.