Lumped model not applying to PCBs
It means you have to worry about transmission lines effects such as ringing, propagation delays, and reflections for RF and high speed digital circuits. It also means you have to worry about leakage currents for high impedance circuits, and interplane and parasitic capacitances for circuits dependent on very small values of capacitance.

You also have to worry about these effects when measuring said circuit since probes introduce elements to the circuit that are not there during normal operation and thus will affects its operation.

Nothing in real life is ever a lumped circuit though so if you keep digging in measurement you'll probably reach a point when the model breaks down.