The study aimed to determine the mistakes of students in operations with algebraic expressions, and their misconceptions that may lead to errors. The study adopted case study method, one of the qualitative research models. The participants were composed of 48 (24 boys, 24 girls) randomly selected among 8th grade students in three different classes from three different schools that were selected via convenience sampling method. To determine students' errors and misconceptions, the "Misconceptions Diagnostic Test for Operations with Algebraic Expressions" was developed by the researchers, considering the curriculum, relevant literature, and researchers' teaching experience. The diagnostic test included 10 open-ended questions. In addition, clinical interviews were conducted with all students on their wrong answers. Results indicated that the most common misconception was that the minus at the beginning of the algebraic expression had no meaning and that half of the students had this misconception. It was also observed that the number of students with the following misconceptions were close to each other: "The operation on one side of the equation should also be applied to the other side so that the equality is not broken", "we should take into account the order of operation priority in integers while writing the sentences as algebraic expressions," and "everything before the parenthesis is distributed to the parentheses." The reasons for the emergence of these misconceptions were found to be epistemological barriers and over-generalization of information in arithmetic.