Teachers' beliefs about students' errors are influenced by structural factors and by other beliefs towards education and students that teachers may hold. The literature on this topic has provided some evidence and some mixed results. Furthermore, some structural aspects related to errors have not been considered in framing teachers' beliefs about errors, such as the use of grades as a classroom assessment practice, which is strongly related to errors in testing situations. Based on these premises, this study aimed to explore teachers' beliefs about errors and the practices teachers report using to deal with students' errors in the classroom and teachers' beliefs about the interdependence between grades and errors. Italian teachers (N = 33) from primary, middle, and secondary schools had been interviewed and the qualitative data were analysed through reflexive thematic analysis. The results showed that, according to teachers, errors acquire different meanings in the learning process, which are related to the roles they play in fostering or not learning. Furthermore, in describing these roles teachers reported to use specific practices to deal with students' errors. Finally, teachers acknowledged that classroom assessment based on grades has a negative interdependency with errors that makes it difficult to present errors as a fruitful part of learning both in learning and testing situations. Our results reveal the ambivalence of teachers' beliefs about errors and shed light on the challenges the grading evaluation system poses to teachers.