Charmed

Look Who's Barking (3x21)

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Phoebe has a premonition of a murderous banshee while adding on to the Belthazar section of the Book of Shadows, thus causing her to think Cole is responsible. But when the sisters cast a spell it backfires and turns Prue into a dog, and when Prue gets hit by a car, Phoebe's remorse causes the banshee to attack her, turning her into a banshee.