In this paper, the author attempts to identify the most common errors that occur in the interlanguage of students at the Faculty of Food Technology when formulating indirect questions in English language. According to Processability theory (PT), language is acquired in a predictable way, in six stages, the last stage being acquiring word order in subordinate clauses, i.e. cancelling inversion. Since interlanguage presents a dynamic language system that retains some features of the first language or generalizes the second language rules in speech or writing, the origin of errors can be found in mother tongue or in the misapplication of the rules when adopting a second language. Although PT is not concerned with the errors made by the second language learners, this paper will try to identify the origin of errors that appear in the students' interlanguage and the acquisition of the last stage, i.e. the word order in subordinate clauses. In that way, it will be determined whether the errors (inter- or intralingual) made by the students prevent them from acquiring the last stage of PT.