The Betrayal of Thomas Becket (1170)
Thomas Becket was the Archbishop of Canterbury and a close friend of King Henry II of England. However, they had a falling out over the rights and privileges of the Church. After Becket excommunicated a number of bishops favored by the king, Henry reportedly exclaimed, "Will no one rid me of this turbulent priest?" Four knights, believing the king wanted Becket dead, murdered him in Canterbury Cathedral. Becket's death shocked the Christian world, and he was quickly canonized as a martyr.
Lesson: Words have power, and can have unintended consequences.