In HBP, chapter 23 "Horcruxes," it says:
"I am confident, however, that the only known relic of Gryffindor remains safe." Dumbledore pointed his blackened fingers to the wall behind him, where a ruby-encrusted sword reposed within a glass case.
But that's NOT the only known relic of Gryffindor's. In GoF, chapter 12 "The Triwizard Tournament," the Sorting Hat sings:
Yet how to pick the worthy ones
When they were dead and gone?
'Twas Gryffindor who found the way,
He whipped me off his head
The founders put some brains in me
So I could choose instead!
So the question is, did JKR mess up? It's possible. Or is she telling us that there could be something else of Gryffindor's out there? Or is just a clue that Dumbledore is only human, a bit of foreshadowing to his death?
I welcome any ideas you might have!