What foreign language learners can do and cannot do could reveal important messages to language teachers about what is happening in their interlanguage. Therefore, this study aims to find out the types and frequency of learners' written errors in the final examination of the English course at a state university research center. The participants were 32 elementary level students who participated in English courses for three months. They were taught certain lexico-grammatical structures in each unit and given a writing assignment related to the vocabulary and grammatical structures taught. In the final exam, there were three different topics and the students were expected to choose one of them and write a paragraph. The participants' errors in the final examination were identified and the interview data questioning their feelings about their writing assignments and the feedback sessions were analyzed with content analysis while demographic variables were analyzed with SPSS 16. The error categories include prepositions, verbs, articles, sentence structure, punctuation, gerunds, pluralism, possessives and word choice. In addition, the categories were divided into sub-categories like omission, overuse and misuse. The results offer important insights into what kind of errors Turkish EFL learners make and their attempts to make inferences about the target language. Interview results also promote various perspectives for English language teachers how to treat learner errors. (C) 2016 The Authors. Published by Elsevier Ltd.