John Rumm wrote:

What any of this has got to do with 12V wiring and plugs though I can't
quite see at the moment.

From the OP's second post I think they discard the live pin and use the
neutral and earth pins when using the sockets in a 12V circuit. So in
theory it should be safe if a 12V appliance is plugged into a mains socket
since N and E should be at the same potential. But in practice it just
needs an error like reversed L and N on a mains socket for plugging a 12V
appliance into it to become very serious.

--
Mike Clarke