What are classical limitation?

Answer 1

1) Classical mechanics does not account for the fact that energy can only be exchanged by tiny packets of a given minimal energy. Therefore in classical mechanics the energy of a system can increase or decrease continuously, while in quantum mechanics it can only decrease and increase by tiny steps.

2) Classical mechanics does not account for the fact that particles behave like waves in some circumstances. Equivalently, one can talk about the introduction of the uncertainty principle that basiquely tells: "the more precisely you will measure a particle position, the less precisely you will measure its speed" and vice et versa. This is not seen as an observational limit due to the weakness of the instruments used or of the human operator, but as a fundamental one: nature seems to be built like that.