The Most Terrible Things Harry Potter Has Ever Done

In Harry Potter and the Sorcerer's Stone, Harry becomes a show-off almost immediately after his first taste of Hogwarts life. While riding the Hogwarts Express to his magical school for the first time, Harry winds up in a compartment with Ron Weasley, his future best friend. He happily enjoys his reputation and celebrity among wizards and witches, eagerly showing Ron the lightning-bolt scar on his forehead when asked. Then the Honeydukes Express treat cart comes around. 

Ron, coming from a large and subsequently cash-strapped family, sadly declines to buy any candy because he's got some kind of boring snack in a plastic bag. Harry, however, fresh from a trip to Gringotts and flush with coin, buys up "the lot"—the entire cart's worth of treats. Squandering cash like that is like throwing around dollar bills in a strip club—arrogant and crass. Nor is it very sensitive to Ron's economic plight. One could argue that it's generous to Ron, but really, doesn't Harry just want to make himself look like a big shot? Also, it's very thoughtless of Harry to buy out the cart—a lot of other kids on that train probably wanted some magic candy, and had probably been looking forward to it all summer.