三人成虎

Chinese proverb, "three men make a tiger", referring to a semi-mythological event during the Warring States period:

According to the Warring States Records, or Zhan Guo Ce, before he left on a trip to the state of Zhao, Pang Cong asked the King of Wei whether he would hypothetically believe in one civilian's report that a tiger was roaming the markets in the capital city, to which the King replied no. Pang Cong asked what the King thought if two people reported the same thing, and the King said he would begin to wonder. Pang Cong then asked, "what if three people all claimed to have seen a tiger?" The King replied that he would believe in it. Pang Cong reminded the King that the notion of a live tiger in a crowded market was absurd, yet when repeated by numerous people, it seemed real. As a high-ranking official, Pang Cong had more than three opponents and critics; naturally, he urged the King to pay no attention to those who would spread rumors about him while he was away. "I understand," the King replied, and Pang Cong left for Zhao. Yet, slanderous talk took place. When Pang Cong returned to Wei, the King indeed stopped seeing him.

-- Wikipedia

In personal development workshops, the saying is, "the one with the mike in their hand is the last to see it." Of doctors and lawyers it is said that one who treats himself, or acts in court for himself, has a fool for a client.