DEUT958 If a single individual testifies before a beis din regarding a sin that someone committed, then he violates the prohibition of לֹֽא־יָקוּם֩ עֵ֨ד אֶחָ֜ד בְּאִ֗ישׁ לְכָל־עָוֹן֙, “A single witness shall not testify about a person regarding any sin or wrongdoing" (Devarim 19:15). Since the testimony of this single witness cannot obligate anyone to pay money or take an oath, nor can it negate another person's good standing, he is considered to have disgraced the other person for no reason, and based in his obligated to punish him with lashes (malkos) for what he did.