Examples of potentially dangerous electrical faults I have come across

Although acceptable in the past, running cables diagonally is dangerous if you don't know where to safely drill!The drill missed the live cable, but this was undiscovered for a number of years.

Cables laid over copper heating pipes, and floor boards sat tight on top. Over time as people walked over the floor, the insulation was worn away until live conductor touched copper pipe.

Shower pullcord. The neutral cable here had not been tightened up enough, and arcing had caused it to overheat, melting the insulation and causing damage to the switch. On this occasion, it was lucky not to start a fire.