I'm not an installer, but as an engineer have sometimes been involved in investigating "odd" faults. Once found a house where one end of a (UK) ringmain ran from the intended 32amp breaker but the other end was terminated in the 16amp intended for the water heater. The water heater cable went into the ring main breaker. The consequence, (un-noticed for several years), was that neither circuit could be switched off until both breakers were off.
