A book for identifying sources of errors, especially for first-year students who occasionally believe they must despair over mathematics. This text aims to reinforce calculus skills and addresses the initial difficulties students face when dealing with terms, equations, and algebraic operations. It serves as an ideal foundation for refreshing school knowledge in addition to mathematical preparatory courses. By describing common mistakes, students learn to identify their own sources of error. This way, they can develop a better understanding of their problems and put them into perspective. Every mistake is not so bad when one understands why it is a mistake.