which awful boy

In Order of the Phoenix, Harry mentions "Dementors" in front of his relatives.

Vernon:'And what the ruddy hell are Dementors?'

'They guard the wizard prison, Azkaban,' said Aunt Petunia.

Two seconds of ringing silence followed these words before Aunt Petunia clapped her hand over her mouth as though she had let slip a disgusting swear word. Uncle Vernon was goggling at her. Harry's brain reeled. Mrs. Figg was one thing — but Aunt Petunia!

'How d'you know that?' he asked her, astonished.

Aunt Petunia looked quite appalled with herself. She glanced at Uncle Vernon in fearful apology, then lowered her hand slightly to reveal her horsy teeth.

'I heard — that awful boy — telling her about them — years ago,' she said jerkily.

'If you mean my mum and dad, why don't you use their names?' said Harry loudly, but Aunt Petunia ignored him. She seemed horribly flustered.

Oho!

Brilliant storytelling, that. We all, like Harry, thought she meant James.

Again and again, Rowling excels in the artist's most mysterious skill: to seem to change the past, by changing our perceptions.