This study was carried out with 210 seventh-grade students and 61 pre-service elementary mathematics teachers. The aim of this study is to scrutinize the skills of pre-service elementary mathematics teachers to determine errors in problems posed by seventh grade students relating to the addition of fractions. The data were collected through two steps. Firstly, the students were asked to pose problems for the operation. Secondly, the Error Identification Test (EIT) including four problems posed by the students was applied to the pre-service teachers. It has been determined that pre-service teachers experienced more difficulties in identifying errors related to units confusion and failing in establishing part-whole relation compared to the other types of errors.