Abdullah bin Masud narrated: "The Prophet (S) prayed five (Rakah) for Zuhr, so it was said to him: 'Has the prayer been added to, or have you merely forgotten?' So he (S) performed two prostrations after he had said the Salam".
The lesson here is that Muslims should always strive to pray with mindfulness and dedication so they can reap its rewards in this life and in the hereafter. Furthermore, it teaches us humility; even though Prophet Muhammad ﷺ was a great leader who was revered by many people, he still took responsibility for his mistakes without hesitation or shame.