Christmas lights often have a plug with prongs at one of the cable, and a socket at the other end. This way you can make a super-long one by just plugging one i

Christmas lights often have a plug with prongs at one of the cable, and a socket at the other end. This way you can make a super-long one by just plugging one into the other. I suspect that someone accidentally just started with the wrong end, and ended up with the plug with prongs at the far end, and the socket at the end by the electrical socket.