The fact that Jaime was right about the old man, with that unspoken suggestion that Brienne kill him. Just think, if she had listened to him, he never would have lost his hand, and who knows if the way Brienne sees Jaime ever would have changed, since it was the bath scene that first started it. And the only reason that Jaime was so vulnerable then was probably because he was half dead and delirous from his injury.
