Based on a corpus of 223 argumentative essays written by English as a foreign language learners, this study shows that spelling errors, whether detected manually or automatically, are a reliable predictor of the quality of L2 texts and that reliability is further improved by sub-categorising errors. However, the benefit derived from sub-categorisation is much lower in the case of errors automatically detected by means of the Microsoft Word 2007 spell checker, a situation which results from Word's limited success in detecting and correcting some specific categories of L2 learner errors.