Another subtle point that was full of win:

"I cannot believe that guy's reaction time," Harry said, brushing off his Cloak as he stood up from where he'd been lying invisible on the floor, unseen by his previous self. "I can't believe his movement speed either. I'm going to have to figure out some way to zap him without speaking an incantation that gives it away..."

The "I can't believe it" is because it's not true - this is the moment he figures out Moody can see him when invisible.