言多必失 Speak much, and there will be mistakes. The Qing scholar Zhu Yongchun wrote in his Maxims for Managing the Home: “In dealing with the world, guard against many words. Many words lead to mistakes.”
祸从口出 Disaster comes out of the mouth. Careless words bring trouble. The Handian dictionary traces it to the Jin writer Fu Xuan: “Illness enters through the mouth. Disaster comes out of the mouth.”
交浅言深 Shallow acquaintance, deep words. To speak of intimate matters with someone one is not close to. The dictionary describes it as speech that goes beyond proper bounds.