This study aims to identify the harmful values experienced by secondary school mathematics teachers in classrooms with gifted students. The participants in the study, which was designed as an interpretative phenomenological design, were two secondary school mathematics teachers. The participants selected using purposive sampling had teaching experiences in inclusive classrooms with gifted students. The data obtained through group interviews, enriched with classroom video recordings, were analyzed by the interpretative phenomenological analysis method. The findings revealed harmful reflections of values contrary to the expectations concerning their positive cultural role. Categories were obtained in the context of mathematics teachers', gifted students' and pupils' values and institutional values. Teachers' harmful values are control, mathematical reasoning, considering students' expectations, equality, relevance (daily life), teacher expectations and esoteric (teaching approach). Gifted students' harmful values are practice, familiarity, memorization, feedback and overconfidence. Other students' harmful values are practice, familiarity, memorization and concretization (learning approach). The institutional values are democracy and inclusive education (teaching approach). The diversity in the sources of values and the resulting conflicts of values have led to undesirable reflections of positive values. Consequently, harmful results were experienced concerning the values alignment, the teachers' skill to manage values and the reflection of institutional values in practice. By revealing the harmful consequences of values, striking implications for teachers, institutions and future research are presented.