Probably a bad switch. Sounds like it's still passing current through. A relay uses a low power circuit to switch a high power circuit. Say you want to run some powerful lights, like in a car. If you use a simple circuit battery>switch>lights, the entire current draw of the lights will go through the switch, including a spike at the switch contacts when the circuits are opened or closed. The more current draw, the higher resistance for a given voltage, which is heat. A higher voltage allows the same current to run more efficiently, reducing resistance. It's all in the equation for W=A x V. If you have a 50W light running on a 6V battery, the current draw will be about 8.3A. That same light running on a 12V battery would only draw about 4.2A. For the higher voltage circuit, you can probably also run smaller wires as the lower current draw doesn't require as large a gauge wire. With the modern regulations for mileage and emissions basically requiring lower weights to meet along with increased electrical demands are big reasons car companies are looking to move to 48V circuits (and went from 6 to 12 decades ago). Oh, relays! So, a relay is fancy switch that uses a small power circuit to switch a large power circuit. The small power circuit is used for the switch, so when you close that circuit it creates an electromagnetic field within a coil inside the relay. That field pulls on the switch for the high power circuit, closing it to allow the larger current that the relay is designed to handle to flow through to the lights. Buying a switch that can handle the same current as the high power circuit of the relay is a lot more expensive and less reliable.
