Illegal Use of Magic
In the magical world, there are many laws dictating the use of magic. Not much is known from the books about the laws and decrees regarding underage wizardry. We do know that every year, students get notices telling them not to use magic during the summer holiday.
Throughout the course of his life, there have been several occasions in which Harry used magic illegally and been notified of it. Twice he has received notices regarding such actions. Once, he left home before any notice could arrive (though presumably there never was one anyway). Before he knew he was a wizard he used magic unknowingly. Not enough information has been given in the book to tell us if those particular instances would be counted as illegal magic or not. Regardless, they'll be included anyway.
- After a particularly bad hair cut, Harry woke up the next morning to find his hair completely regrown.
- Aunt Petunia tried to put an old jumper (sweater) on Harry, but it only got smaller and smaller the harder she tried. She decided it must have shrunk in the wash.
- He turned his teacher's wig blue.
- While Harry was a student at primary school, he was often ridiculed and teased, especially by Dudley and his gang. Once, while trying to escape them, Harry found himself on the roof of the school.
- Harry freed a boa constrictor at the zoo by making the glass disappear.
Before the start of the school year, a house-elf appears in Harry's house. He uses a Hover Charm to levitate a pudding (and subsequently drop it) on Harry. Apparently the monitoring done by the Ministry detects when and what magic was used, but not who the caster really is. Harry is blamed for the Hover Charm and warned to not cast anymore spells or he will be expelled. He successfully makes it back to Hogwarts, however.
During the summer break of Harry's third year, Aunt Marge comes for a visit. Her insults towards his family take him to his breaking point. He accidentally blows her up, and she goes flying away into the night sky. Harry leaves immediately, and takes the Knight Bus to the Leaky Cauldron. He meets with the Minister of Magic, and learns he won't be expelled.
A dementor attack in Little Whinging forces Harry to abandon the Underage Decree and use the Patronus Charm. The charm is successful in driving away the Dementors, but the Ministry of Magic is alerted of the spell. Not only was it underage magic, but the magic was done in front of a muggle as well. Harry is at first told his wand will be broken, but the Ministry backs off after Dumbledore confronts them. In the end, Harry is forced to attend a court hearing to determine his fate. He rightfully escapes any punishment, and is allowed to return to school.