Dobby's Bludger
From Harry Potter Wiki
The house-elf Dobby tampered with a Bludger during the 1992-1993 school year. Its purpose was to injure Harry Potter so that he would be (hopefully) sent home from Hogwarts. The Chamber of Secrets was active and petrifying students at the time. The Bludger caused the Gryffindor Beaters to protect Harry and not watch out for the rest of the team. Despite their attempts, the Bludger succeeded in breaking Harry's arm. Dobby later apologized for setting it on Harry.
