The fear of turning to minus chas vesholom is stated in the 3rd parsha of shma. Every day is a challenge and the greater the person the greater the chance. But we rely on chezkas Kashrus of yesterday as a predictor for today. Even a shochet that messed up had a chazaka yesterday. Safek mamzer is permissible min Hatorah and the Rabbis were not strict like other sfekot.
