Surprised component
Technician thought he had broken the test rig because there were no signs of power. Unplugged the board being tested and checked: nope, the test rig was fine. Connected everything back up, and still seems like no power.
After sitting back and thoughtfully stroking his chin the top of the line driver that was plugged in backwards audibly ricocheted off the 30 foot (9 meter) ceiling. Then the lights all came back on the test rig.
He said he hadn't missed getting an electronic caste mark by more than a couple of seconds.