Once upon a time, he hadn't caught her before she fell.
He'd only watched as she tumbled, crumbled, and hit the ground, and there was nothing that he could do.
It was the first time, and he hoped that maybe, just maybe, he could ratchet up that saving-people-thing to the point where he'd never have to watch her fall again. He'd already lost too much that way.
He said that he was sorry, even though he knew that she wouldn't listen. She'd never blame him for anything. It was a while before Harry figured out why.
The next time Hermione fell, he fell with her, a tangle of limbs and lips and other body parts that he wanted to touch all at once, and she was the one who apologized, but only for taking so long to get around to it.