Planck, in his treatment to solve blackbody radiation, considered that the electromagnetic waves inside the cavity of the blackbody are standing waves due to oscillating charges on the wall of the cavity. At equilibrium, the frequency of oscillation of the oscillating charge is equal to the frequency of the electromagnetic wave produced by it i.e. at equilibrium, energy of the oscillating charge is equal to the energy of the produced electromagnetic wave.
He proposed that, the energy of the oscillating charge is integral multiple of $h\nu$ i.e. $\epsilon_n=nh\nu$ where $n=0,1,2,3,4,...$
But, we know that quantum harmonic oscillators have energy $\epsilon_n=(n+\frac{1}{2})h\nu$ where $n=0,1,2,3,4,...$
Then, are not the oscillating charges on the wall of the Cavity supposed to be quantum harmonic oscillators?