This study aims to describe student errors and the factors that cause student errors in solving mathematical reasoning topic questions according to Castolan Stages. This research uses a qualitative approach with descriptive methods. The research subjects were 3 class XII high school students consisting of 2 female students and one male student. The research instruments used in the research were written tests and interviews. The results showed that students made three errors in solving the mathematical reasoning questions: conceptual, procedural, and technical. The causes of these three faults differ. As for student errors, (1) conceptual errors because students choose the wrong formula or formulas, so they don't answer questions; (2) procedural errors because students don't work on questions according to the steps requested, so they don't write down what was known and what was asked; and (3) technical errors. The cause of conceptual errors is pupils not understanding the subject and forgetting the formula. Procedure errors occur when kids don't know how to solve problems and aren't used to writing down what they know and what they're asked. Technical errors occur when pupils rush to solve problems and don't check their responses.