When the house settled at night, the stud wall would flex, the crack would open by a millimeter, and the circuit would break. When the sun warmed the house, the plastic expanded, the crack closed, and the light worked.

See the Electrical Crack: Why the Smallest Flaw Can Spark the Biggest Disaster

I walked in and killed the main breaker. I pulled the switch out and held a magnifying glass to the side of the nylon body of the switch. There it was. A crack no wider than a human hair, running from the hot screw to the ground yoke.

Electricity is invisible. Safety is visual. Don't just look at the wires; stare at the cracks.

Here is why finding that crack before the power turns on is the most important skill in the trade.