A high percentage of students entering university have mathematical errors which are shown during the process of building the new content and which affect their academic performance in the discipline. This article presents the results of a study to analyse the errors made by the first-time student in the mathematics diagnostic test at the National University, Costa Rica, in 2017. Research is qualitative, explanatory and descriptive. The test consisted of 60 single selection exercises. Each disclaimer was required to record in the screening brochure all the procedures carried out. A random sample of 100 tests was selected. For each item, a choice was made of the tests where the student had carried out a procedure and the errors were classified according to their origin: mathematical language, spatial information, incorrect inferences or associations, retrieval of a previous scheme, incorrect or accidental calculations, construction deficiencies or lack of prior knowledge. In addition to presenting errors in the abovementioned categories, the person studied presented errors in mathematical content such as identification of priority of transactions, use of parentheses, concept of absolute value, transactions with polynomies, remarkable formulas, power laws, factoring, among others. The results show that students presented various errors and shortcomings in mathematics, so there is a need to put forward strategies, at university level, to correct this situation and to ensure that every dissender has the necessary bases to successfully face courses in the discipline.