Harry frowned. "Oh, maybe. I didn't think of that… is there a transfiguration spell which turns a feather duster into a paintbrush? That would help here."
"Oh, hold on, I think I saw one of those," Dean volunteered, flipping through his own copy. "Uh… there."
Harry wrote down the words of that spell, looked at his spell-chain, then frowned. "I think I must have copied one of these down wrong."
"Let me see?" Hermione asked, comparing the spell chain to her own – all finished already, of course. "There's a difference there… hold on… oh, I see. No, that spell won't fit there, I think you copied the wrong one down."
Sighing, Harry took the paper back and crossed out the offending spell. Finding the page again, he saw that he'd put down the wrong one on the page – instead of turning a duck into a pillow, he'd put in one which turned a weasel into a wastebasket.
"That's better," he said, substituting the correct one in. "Thanks, Hermione."