This study attempted at classifying common errors found in the written performance of lower- and upper-intermediate Iranian EFL learners. It engaged a rich corpus of EFL writing samples collected over a course of 20 years (between 1992 and 2011) from lower- and upper-intermediate EFL learners studying at various Iranian universities to provide a precise taxonomy of errors in basic-level EFL writing (i.e., single paragraphs and five paragraph essays). A total of 3157 sophomore EFL learners were included in this study, and from each of them five writing samples were collected. There was a total of 15785 texts in the corpus which contained a total of 5,150,205 words. Corder's (1981) framework for error analysis was implemented, and it was found that basic-level EFL writing errors could best be classified into three major categories: structural, discursive, and cognitive. Classroom procedures and teaching techniques that can help both teachers and learners to overcome the identified error types are discussed.