What does harry think must be a mistake and how does hagrid convince him otherwise?

Harry thinks there must be a mistake regarding his acceptance into Hogwarts because he believes he is just an ordinary boy with no special abilities. Hagrid convinces him otherwise by revealing Harry's true identity, including his famous past and the fact that he is a wizard. Hagrid also presents Harry with his acceptance letter, emphasizing that he has been chosen for a unique and magical life at Hogwarts. This revelation helps Harry understand that he is indeed special and worthy of his place in the wizarding world.