I went to a small job today to change a sensor for an outside light. I expected it to be straight forward. I started by isolating the supply, which ended up taking longer than expected as it needed 2 circuit breakers to be off in order to isolate it. I explained that i would need to remedy this first before changing the sensor but the customer didn't want to pay for the extra work so that was the end of the job. My question is, how does an outside light or indeed anything get fed from 2 circuits in the first place? It must be working from one circuit so why does someone feel the need to add it to another circuit as well? How could this fault occur?