oh, oh! Cliche -- A potions accident leaves snape as a cat until who-knows-when. Dumbledore asks Harry, after every (other?) teacher at Hogwart's, to care for him until the solution is found or the potion reverses itself. Snape is, of course, no more friendly as a cat than as a person. Bonus points if Harry doesn't tell Ron an Hermione what's happened, to spare snape the embarrassment.