It's hard to pick the silliest thing, but all the really bad ideas come from electricians.
I had a maintenance electrician for an oil company give me a stick to wedge into a contactor to keep an oil well running while we waited for a new coil.
Another electrician ran two 120 volt feeds from a panel to a wall pack - one for the built-in photocell and one for the light.
Then there was the electrician who threw the thermostat away when he installed a baseboard heater. He thought it was self-regulating.
My favorite is the electrician who thought that a plc analoq card failed because the transmitter was improperly calibrated. A tank was 50% full, the transmitter was showing only 40%. He said the card failed when it tried to reconcile the difference.
