In early 1980s, I was a member of a team working on a nuclear magnetic resonance system, which required a very stable 100V 10A power supply for the magnetic field.
The stability requirement was a tough ~1ppm of noise and hum, and my colleague who was designing the power supply built a coarse 3-phase thyristor regulation, followed by a smoothing linear regulator.
To his big disappointment, those thyristors, even when abundantly over-dimensioned, kept misfiring, and the high inrush current into a large capacitor bank kept melting the semiconductors down, and causing in turn a short on the 3-phase mains, triggering the automatic circuit breakers, both on the PS rack, and the main ones, shutting the whole department off. It was happening several times a week, so it soon became rather annoying.
Finally to everyone's relief, my colleague has managed to reduce that inrush current, and also obtained some 70A rated thyristors, which cured the problem.
As the delivery day of the NMR system approached, we invited a professional photographer to take pictures for the documentation, and that's where I got my bad idea...
Demo day arrives
On the day before the demonstration for our customer, we were all busy with the last checks and preparations, and the PS designer was naturally rather nervous himself, he was sitting by the PS and an oscilloscope with the probe attached to the output voltage, the input AC coupled and the sensitivity maximized to show the line ripple, fine- tuning some trimpots, and watching carefully the changes on the output waveform.
I approached quietly behind him, with a photographic flash light fully charged, and as he reached once again for one of the trimpots I pressed the flash button. The flash light fired with a silent "PUFF".
For a few microseconds the time stood still. Then as in a slow-motion my colleague jumped up, the screwdriver which he was using to adjust the trimpots went left, the chair flew backwards, his glasses to the right and as he was falling on the floor he reached for the power switch and pulled it down.
The room burst into laughter.
After we all calmed down again my boss approached me and told me that my little joke could have had some very serious consequences if, by chance, my colleague dropped the screwdriver into the PS.
"I know, boss," I said, "so I gave him my plastic screwdriver last week!"