The Itchy & Scratchy & Poochie Show - S8-E14
Corrected entry: When Itchy shoots the apple through the arrow at Scratchy's head, it punctures a vat of carbolic acid which spills onto Scratchy, turning him into a skeleton. Carbolic acid, or phenol, is poisonous but not corrosive.
Correction: Bear in mind that this is the episode in which nerds are mocked for nitpicking cartoons.
Correction: An Itchy and Scratchy cartoon is entrely exempt from having to follow the rules of reality. OR, you could argue that since within the Simpson's universe the writers of Itchy & Scratchy have always been shown to be pompous and unimaginative college boys that don't do their research.