Over the years I have seen some domestic installations that could have quite literally killed someone. These have usually been done by the householder or a ‘mate’ who knows a bit, but not always.
Some examples of which would include wall lights wired in bell wire and supplied from the rooms ring main which is protected by a 30A fuse. If a fault occurred with the light the wire would disintegrate before the fuse would blow.
I have tested metal fittings that seemingly are connected to earth and found they are not. Further investigation showed that not only had the supplying cable been drilled through twice, once severing the earth or CPC wire. Another screw was only insulated from the live cable by the rawl plug!
Cables run under the carpet are another common find, very dangerous when they cross or run along the carpet gripper!
On two occasions entire houses not connected to earth due to a fault with the suppliers earth connection.
My personal favourite though is a standard internal 13A 2 gang socket screwed to the outside of a house to use with the lawnmower, no RCD, no weatherproofing, lethal!