The present study investigates the writing errors of ninety Saudi non-English major undergraduate students of different proficiency levels from three faculties, who studied English as a foundation course at the English Language Center in the College of Languages &Translation at King Khalid University, Saudi Arabia in the academic year 2016-17. The findings reveal that the common errors the Saudi EFL students make in writing English paragraphs fall under four categories namely grammar, lexis, semantics and mechanics. Then it compares the categories, types and frequency of errors committed by these three groups of students. Among these categories, grammar has been observed as the most error-prone area where students commit errors the most. The study also posits that among the three groups, the students of the College of Medicine make the minimum errors in all the types and the highest number of errors is committed by the students of Engineering College. The College of Computer Science is in the second position in making errors. The frequency of error types is also found different among these three groups.