Here's a situation: a paladin without a helmet attacks a vampire. The vampire swings its scythe across the paladin's face, but the scythe isn't disinfected, and undead creatures don't have hair (they're skeletons) or their hair falls out. So, the paladin's facial hair ends up on the vampire, and the vampire turns into a mouse, becomes a mist, or starts levitating. Then, back at its lair, it drinks a blood cocktail, regenerates, and the vampire has hair again.