I like this one from the Weasley twins:
"Oh, are you a prefect Percy? You should have said something we had no idea."
"Hang on I think I remember him saying something about it, Once..."
"Or twice-"
"A minute-"
"All summer-"
And this one from Ron:
"That'll change the world, that report will, front page of the Daily Prophet, I expect, cauldron leaks."