The answer is that there are "physics quantities" that do not obey a superposition principle. The energy density of the electric field is proportional to E2 so if E=E1+E2, the energy density is not the sum of the energy densities due to E1 and E2 separately.

When can superposition not be used?

If at least two independent sources have the same frequency (for example in power systems, where many generators operate at 50 Hz or 60 Hz), then superposition can't be used to determine average power.

What are the two conditions to be satisfied if the principle of superposition is to be applied?

All the components of the circuit must be bilateral, meaning the current will remain the same for opposite polarities of the source voltage. Active components and passive components may be used.