This study sought to solve the puzzle of persistent occurrence of mechanical errors in pre-service primary school teachers' writings. Data consisted of short compositions from four intact classes (n=75) of second-year university students at the Higher Institute of Arts and Crafts of Tataouine (ISAMT), Tunisia. Results of data analysis revealed a high rate of mechanical errors and indicated that negative transfer and fossilization were major obstacles in the process of mastering L2 writing.