This qualitative study examined the errors of EFL secondary school students in the early education period before attending colleges and universities. Error analysis was employed to examine written errors in the final examinations and to determine the causes of these errors. More than 5000 errors were collected from the exam papers of 904 students from two secondary schools. This study found four types of errors in the writing of Saudi EFL secondary school students: grammar, mechanics, lexis, and semantics. The tracking of these errors assisted in the analysis process to verify these factors. Most written errors were caused by one of the following factors or a combination of them: Arabic language transfer, English language transfer, teaching-related issues, and student needs. These findings provide educators and policymakers with sufficient data on common challenges in language teaching and learning. They can propose suitable solutions to improve their teaching approaches, learning strategies, and curriculum designs.