James Phelps accidentally cracked director Mike Newell's rib during a fight scene on set for The Goblet of Fire.
Watson fell down the stairs during Hermione's dress-reveal scene at the Yule Ball.
Maggie Smith chose Grint to dance with her during the Yule Ball rehearsal, and the rest of the scene was ad-libbed.
No one, including the directors, knew ahead of time what would happen to their characters, except Alan Rickman, who convinced Rowling to tell him about Snape's fate.
Jason Isaacs, aka Lucius Malfoy, originally auditioned for the role of Gilderoy Lockhart. After being asked to read for Malfoy, Isaacs read his lines so bitterly that there was no contest for who should play Draco's father.
In a deleted scene, Isaacs accidentally cut Felton with the teeth of his walking stick. "I went, 'Tom, I'm so sorry, I didn't realise how sharp they were and how heavy it was,'" Isaacs said, "And he [tearfully] went, 'No, no, it's all right. It was good for the scene.'" Felton jokingly added, "He'd be this evil father and then they'd say 'Cut!' and he'd come and give you a cuddle."