Blackbody Radiation
The true beginnings of the quantum theory lie in a strange place: the frequency spectrum emitted by a solid when it is heated (“blackbody” radiation).
Experimental measurements: the frequency spectrum was well determined.. a continuous spectrum with a shape that depended only on the temperature (light bulb, … )
Theoretical prediction: Classical kinetic theory predicts the energy radiated to increase as the square of the frequency (Completely Wrong! - “ultraviolet catastrophe”).