Everyone makes mistakes. Some are just funnier than others.
About a year ago, the A/C unit that feeds the printing area in our facility stopped working. I immediately grabbed my meter and proceeded to check for available power at the unit. Although the meter showed there was no power, the indicator lights were still lit. I assumed a capacitor somewhere in the system was keeping the lights on. However, when I switched the disconnect to the off position all the lights went out. Assuming this was just a coincidence, I tried turning the switch on and off several times. Every time I turned it off the lights went out. This whole process took about 15 minutes, and in the end my meter still showed no power. As I stood there scratching my head and looking at the meter, I finally realized the problem. The meter I use at home was different than the one I use at work. I had the leads set up to test for resistance instead of voltage!
My lead electrician, along with two helpers, was installing 11 smoke detectors in a veterinarian's office on a Saturday after hours. He called me to say that he couldn't get them to communicate with each other. He was using 12-3 with ground MC cable. I asked him where he tied the red wire in at the junction box above the breaker panel. After a long moment of silence, he said he remembered connecting two black wires with a red one. He had tied the signal wire right in with the 120V power conductor! After smoking the smoke detectors on the 9VDC signal side, we ended up replacing all 11 units. But this time he made sure he capped off the red wire.
Illustrations by Clint Metcalf