Snape has cast Gluteus Maximus. It's his favorite spell, because look at dat ass. Anyways, having read the Harry Potter series myself, I am a fan of J.K. Rowling's work... and a critic of it as well. No writer is perfect, but something feels wrong when you don't give your characters the freedom to change, even if it does affect the original plan. Of course, as the writer, you are essentially "God." And your say is the final say.