In Harry Potter and the Philosopher's Stone, Harry looks into the Mirror of Erised and identifies the people he sees within as his family.
She was a very pretty woman. She had dark red hair and her eyes — her eyes are just like mine, Harry thought. […] The tall, thin, black-haired man standing next to her put his arm around her. He wore glasses, and his hair was very untidy. It stuck up at the back, just as Harry's did. […]
And slowly, Harry looked into the faces of the other people in the mirror, and saw other pairs of green eyes like his, other noses like his, eve a little old man who looked as though he had Harry's knobbly knees — Harry was looking at his family, for the first time in his life.
Did the Mirror of Erised show what his family actually looked like, or merely what Harry imagined that they would look like? Harry had already learned a bit about his mother and father, but not about his other relatives (such as the knobbly kneed one), so it's unclear to me if the Mirror or Harry is filling in the gaps.