I was just watching property ladder, and the guy said that after they
had just installed the lighting circuit, they had found out it was the
wrong sort of cable.

Can you think of what he meant? They didn't actually say any more on
the subject - which is really annoying for a viewer, if we are not to
make the same mistakes they should bloody well say what the exact
problem was.

The only things I can think of are low voltage lights, and the wrong
colour coding of cables, which changed a few years ago. Can you think
of anything else it might have been?
Thanks