If nowadays there is a clear tendency to consider the errors made by learners in their process of language learning not as a negative aspect but as a natural step in the development of their language skills, in the past teachers pondered the errors committed by students as something unfavourable, something to prevent from occurring. However, in recent decades, researchers have come to consider errors as the evidence for a creative process in language learning. Therefore, the main objective of this paper will be to characterise and classify the errors made by undergraduate engineering students in a public Spanish Polytechnic University over the last two academic courses. We will try to categorise those errors taking into account their source but following one of the distinctive taxonomies proposed by Dulay, Burt, and Krashen in “Language Two” (1982): the comparative taxonomy. The comparative taxonomy of errors, upon which we will be basing ourselves on the present research, arranges errors into the following four categories: interlingual errors, developmental errors, ambiguous errors, and other errors. Consequently, what we shall be demonstrating along this article is that, contrary to the opinion of previous studies and researchers, the most frequent category of errors, at least for these technical students, is the interlingual category of errors. And in order to be able to lead this research, a corpus of a total of 72 essays was examined, comprising their written productions in the task assigned throughout these last two years that these students had to take on.