Nearly extreme black holes which attempt to regrow hair become bald again

Black holes 'have no hair': no attributes that can be used to tell them apart. Extreme black holes (spinning at maximally allowed rate) can have an additional property, permanent hair that is made of a massless scalar field. Nearly extreme black holes (like Gargantua, the black hole featured in the movie "Interstellar") have hair that is a transient phenomenon: nearly extreme black holes that attempt to regrow hair will lose it and become bald again.