Harry Potter and the Philosopher's Stone/Harry Potter and the Sorcerer's Stone - Quidditch Scarf
On the cover of the book Harry is standing on platform nine and three quarters, wearing his quidditch scarf, looking amazed at the Hogwarts Express and experience of the magic entrance.
Harry didn't know what house he was in until he got to Hogwarts so he couldn't be wearing his scarf BEFORE he got on the train!
The covers of all the books have a jumble of illustrations of events or people in the books, AND the illustrator is merely depicting her own representation of events, the drawings are not a part of the author's story.
Very true (although not necessarilly a part of the quidditch attire; maybe school uniform... though he wasn't in a House yet, and it's Gryffindor colors... right.... illustrators don't write books). Besides, did you notice that the Hogwarts Express is right next to a regular train without any barrier (magical or otherwise) between them? That's a more critical illustration error, I think. But that is, of course, just my humble opinion.
These kind of mistakes have been made a few times, did you notice the movie? The same thing happened in the movie, everyone was wearing house colours. They weren't even wearing correct uniform! (A tie and shirt, it was supposed to be a black robe).
I take it not as a slipup from J.K. Rowling but as a careless mistake from the publisher and dress manager.
What can you do?
The illustration is usually just what Sway999 said: a jumble of depictions that do not necessarily correspond chronologically to the book's story but do indeed summarize the events that take place within it.
Has anyone bothered to think that the illustrator may have based the cover illustrations on Harry's RETURN journey, when he arrives BACK from Hogwarts to platform nine and three quarters, when he has finished his first year at hogwarts, and knows what house he is in? just thinking outside the square.....or platform!!!