In this article we report an investigation on students' understanding of: 1) unit vectors and, 2) scalar multiplication of a vector. We administered two different tests to a total of 850 students after taking their first introductory physics course on mechanics at a large private Mexican university. The first test about unit vectors was taken by 270 students and the second test about scalar multiplication of a vector by 580 students. In the first part of this article, we analyze students' difficulties sketching the unit vector in the direction of a vector in the Cartesian coordinate plane. In the second part, we analyze students' responses in two types of problems: positive and negative scalar multiplication of a vector. In both parts of this article we describe frequent errors that have not been reported in the literature.