Measuring resistance between Line (L) and Neutral (N) in a circuit should ideally yield zero ohms, but readings of 0.3 ohms can occur due to the source impedance being in parallel with the circuit impedance. This discrepancy indicates that the circuit is not completely isolated, as even when the black wire is unplugged, some resistance can still be measured. It's important to ensure that measurements are taken with the circuit disconnected from the power source for accuracy. The circuit's functionality, such as whether the bulb shines or not based on switch positions, also confirms proper operation. Understanding these concepts is crucial for effective troubleshooting in electrical circuits.