"Nobody is perfect, and there have definitely been times in the past when I have witnessed others make mistakes. When it comes to someone who has authority over me, despite this I would still report the mistake so the project stays on course and reaches its desired end. So that I don't embarrass anyone, I would request a private meeting and bring up the issue in a friendly and truthful way. In addition, I would frame it so the person knows that we share a mutual goal of completing the project successfully and that's why we need to correct the mistake."
