Harry Potter and the Sorcerer's Stone had a spell plot hole involving Hermione Granger (Emma Watson) that's easy to miss. The first Harry Potter movie introduces viewers to Harry (Daniel Radcliffe), Ron (Rupert Grint), and Hermione as they navigate the increasingly complicated Wizarding World of J.K. Rowling. But for a brief moment, the movie seems to forget one of Hermione's defining traits.
When he boards the train to Hogwarts for the first time, Harry meets Ron (who has dirt on his nose). Later on, Hermione joins them in the compartment and sees Ron trying out a spell. She taunts him to show his skills; he fails, then she asks, "Are you sure that's a real spell?" She goes on to say she's only tried a few simple spells, but they have all worked for her. Then, she sees Harry's broken glasses and wastes no time demonstrating her magic skills by casting Oculus Reparo, repairing Harry's glasses, and impressing both Harry and Ron.
There's a problem with Hermione casting spells outside Hogwarts. This is something that can get students expelled from the magic school, and for Hermione, getting expelled would literally be worse than death. In the same film, Hermione tells Harry and Ron: "I'm going to bed before either of you come up with another clever idea to get us killed or worse… expelled!" Moreover, Hermione seems well-prepared on any subject and lectures the boys several times throughout the film. The fact that she has been practicing spells at home (and then casts one on the train to Hogwarts) simply seems out of character for Hermione Granger.
Hermione's carelessness does not end with the first Harry Potter movie. In Harry Potter and the Chamber of Secrets, she casts Oculus Reparo again on Harry, this time in Diagon Alley. If Hermione perhaps didn't know about the law stating no students are to cast magic outside Hogwarts before her first school year, she surely did by the start of the second one. It's doubtful Hermione simply wasn't aware students aren't supposed to cast spells in the Muggle world, even during Harry Potter and the Sorcerer's Stone: after all, her own parents are Muggles. Hermione grew up in a world where she had to conceal her magic, even though her parents were supportive of it.
There is only one defense argument for Hermione casting spells outside Hogwarts in this case. It can be argued that this law was created in order to protect the International Statute of Secrecy. Therefore, both the Hogwarts Express and Diagon Alley are safe places to cast spells. Granted, Hermione told Ron and Harry that she practiced spells at home, too. If her worst fear was getting expelled (at least during her first year), casting magic outside Hogwarts was very much going against her personality. It's a detail Harry Potter and the Sorcerer's Stone didn't explain, which is pretty strange, considering this is the very first movie where the characters' personality traits are introduced and defined.
