A few years back I received a call from a long time client, she was getting shocked whenever she touched the sink and stove at the same time.
I started my troubleshooting by interviewing my customer. When did the problem first start? What other events took place during that time? Is it an intermittent problem or does it occur every time?
Armed with useful information I proceeded. I verified that there was indeed a difference of potential between the metal frame of the stove and the stainless steel sink. There was approximately 90V. I then checked the metal frame of the refrigerator to the sink and once again had a 90V reading. I checked between stove and refrigerator and found 0V. Now knowing the voltage source was the sink, the possible causes was narrowed down. I went to the storage room to check on the water heater. I found the green, bond wire disconnected from the water heater. It was sticking straight up in the air with no contact what-so-ever to the water heater. There was also a small leak in the top of the water heater which was running down over one of the thermostats. This caused the casing to become energized, which in turn energized the water pipes. I turned off the power and water to the water heater and had the customer call a plumber to either repair or replace the water heater. I installed a ground wire from the cold water pipe to the grounding electrode and when the water heater was replaced, I returned to make the electrical connections to it.
Had this woman taken a shower before making the call to me, I might not have ever gotten the call.
Another time I had a friend that was building a custom home. It required 2 well pumps, one for house use and the other for irrigation. I wired both pumps to individual 220V circuits.
The irrigation company came in to install a Rainbird sprinkler system. The control panel had a small transformer to provide 24V for the remote valves. The problem was, the primary side of this transformer was 120V. The irrigation crew decided to tap off the pressure switch for the pump circuit to provide this 120V. They attached the hot wire to the line side of the switch and to get a neutral, they attached to the grounding terminal.
I returned to start the wiring for the 30' X 60' workshop and noticed the new sprinkler control panel. I decided to check it out. As soon as I opened the front cover and read the transformer I knew they had violated code. I opened the pressure switch and found what I described earlier. I disconnected their wiring and called the irrigation company to explain to them how their improper wiring methods could kill someone. I suggested they find a 220V → 24V transformer because the homeowner would not allow them to reconnect the 120V transformer.
Simple errors in wiring can become life threatening incidents later. Proper training and installations save lives.