Harry Potter and the Prisoner of Azkaban - Werewolf Sirius?
In the Prisoner of Azkaban, after all of the events in the Shrieking Shack have taken place and when Lupin is turning into a werewolf back at Hogwarts, Sirius transforms and goes to fight him off and protect Harry, Ron, and Hermione. Shortly after, Harry finds Sirius covered in wounds. The only thing is... if Lupin gave him those wounds, wouldn't that make Sirius a werewolf too? Hard to believe that after all that fighting, Lupin wouldn't have gotten one good bite in. Interesting idea...