Kiddushin 66 - Credibility of Witnesses
If a single witness says to a person, "Your ox was sodomized," and it is therefore unfit to be a sacrifice, and that person remains silent, the witness is believed.
King Yannai was incited by Elazar Ben Po'ira against the sages of Israel. Yannai wore the golden plate of the High Priest, and one sage protested that Yannai was an unfit Kohen. Elazar suggested to kill all sages. But what of the Torah? - Here it is in the corner, anybody who wants can learn! Yannai executed the sages. Why was Yannai fit? The witnesses to his unfitness were later established as false witnesses.