Another nice thing can happen when lights and receptacles in one room are on different circuits. Once I had a broken light. I started to trace the fault and screwed out the fuses for the receptacle in that room. I didn't have a phase probe, only a 2 pole voltage tester, so I only checked if the receptacle was dead, which was correct. Then I started working on the switch (carefully, to my luck, not touching any bare wires). After I had put everything back together I tried the switch (sometimes the cover of such old switches blocks the switch if it doesn't sit straight, so I try them before restoring power) and was pretty surprised when I was greeted by the light of a 100W bulb. The receptacle was fed from the adjacent room and was on a different circuit.
That's why I prefer having lights and power on one circuit.
