By far the worst example of this and likely the moment he regretted most, was when he used an unknown incantation on Draco Malfoy, almost killing him. Harry had read about the spell Sectumsempra ...

Despite admiring the Half-Blood Prince for his potion-making advice, he discovers a darker side to him when he uses Sectumsempra, a spell from the book, against Draco Malfoy. Despite his friends ...