Having too much pride or confidence will cause one to make mistakes that lead to a setback or failure. The phrase is adapted from a verse in the book of Proverbs in the Bible: "Pride goeth before destruction, and a haughty spirit before a fall." A: "He's so unwilling to delegate any responsibility to anyone else that I'm sure he's going to end up crumbling beneath all the pressure." B: "Well, pride goeth before a fall."