The aim of this study is to examine preservice mathematics teachers' proving skills in an incorrect statement. In this way, it was tried to examine their reasoning and proving skills about the correctness of the given mathematical expression. The case study, one of the qualitative research designs, was adopted in the study. The participants of the study are 47 preservice mathematics teachers studying in the fourth grade. The data were first collected in writing with the question of "sum of triangular numbers". Afterwards, semi-structured interviews were conducted with five volunteer preservice mathematics teachers. Content analysis was used in the analysis of the written data. The findings showed that the preservice teachers did not question when expressed with "prove or show that it was true", they did not have knowledge about alternative proof methods, and they insisted on using the proof methods they were familiar with. In the light of the findings to be obtained, alternative proof methods have been tried to be presented.