Abstract: In this study, we used the concept “didactical contract”, which was introduced by Guy Brousseau in 1980, as a tool for finding out the errors of students. In order to illustrate this approach, we used the type of a task: “In the space Oxyz, write the general equation of a plane (P) passing through point A(x0, y0, z0) and parallel to two straight lines: (d1) and (d2) ((d1) and (d2) are not parallel to each other)” which was mentioned in mathematics textbooks of Vietnam. We formulated the hypothesis H: there exists a rule of didactical contract: When solving the problem, students don’t verify whether (d1) and (d2) are parallel or not; therefore, students will commit errors in the case of (d1) and (d2) are parallel. The results of study showed that many students committed errors in solving the exercise because of the above didactical contract.