The Mirror Of Erised
In the first book and movie we saw the Mirror of Erised, that showed the deepest desire of the person who looked into it.
Harry saw himself with his family while Dumbledore said that he saw himself opening a Christmas present containing socks.
Harry felt that the Headmaster was lying but did not ask him about it. Later, Rowling revealed that like Harry, Dumbledore also saw himself reunited with his family.