I’ve got a handful of doozies, but this one is probably the most interesting:
I used to work at a Navy contractor that made sonar devices (sonobuoys, if you recognize the term, as well as other sea-going sonar test articles). We were R&D, and typically our projects would last a year or two, produce only a handful of buoys, and include a number of interim tests, usually at sea.
On this particular project, I was the only mechanical engineer, so I was in charge of all the non-electronic things: surface float, pressure vessels, cabling, connectors, and the battery pack. The battery pack was a pretty big item, and the battery pack consisted of about 400 F cells (four F cells make up a lantern battery) stacked up and wired together. Not a bad design actually: cheap, modular, and reliable.
Anyway, we had a big sea test scheduled, and, like usual, everyone was working late the night before the hardware had to ship. I had all the mechanical stuff buttoned up, and the electronic guys were just finishing programming their boards. Last thing to do: “Hey, let’s unplug the electronics from the bench power and make sure it runs OK off the battery pack.” So we trundle in the battery pack and screw in the connector to the boards…
And thirty seconds later the electronics boards erupt in flames. Since there was no off switch, we all pulled a Three Stooges act, running in circles and waving our hands, until someone was bright enough to yank the correct wires. Well, the electronics are toast, and we can’t make the sea test, so we have to postpone at the last minute, and we’re behind schedule, and we’ve got a lot of repair work and troubleshooting to do.
So we troubleshoot. Turns out the battery pack was supplying negative voltage instead of positive, which fried certain critical components. And the reason for that was that the “industrial” F cells I used looked just like a regular battery: a cylinder with a button on the end, except that, unlike a “normal” battery, the button end was negative, not positive. I had just assumed the button was positive, and never, ever bothered to check. Easy enough to alter the wiring now, after umpty-thousand dollars of one-of-a-kind electronics vaporised.