Harry Potter and the Chamber of Secrets
by J.K. Rowling
Mafalda Hopkirk is a witch from the Improper Use of Magic Office who writes to warn Harry against using magic outside of school so long as he's underage (i.e., under seventeen in the wizarding world). She seems to believe that Harry is the one who cast the Hover Charm that destroyed Aunt Petunia's dessert when the Masons came to dinner, but in fact, it was Dobby the house-elf. Uncle Vernon uses this letter to justify his decision to lock Harry up: after all, Harry isn't allowed to magic his way out, and the Dursleys will be able to keep Harry away from Hogwarts.