Biggest electronics mistake you ever made?
Not sure if this is the right kind of question, but I'm feeling bad, because after painstakingly assembling 8 surface mount boards with about £5 worth of components on each board and carefully gluing them in place (they're Hall effect detectors for a model train turntable) I used a 25-pin d-sub connector to connect them to power and logic inputs.
I forgot that d-sub pin numbering doesn't map directly on to ribbon cable, so fried at least half of them by ramming 5 volts into an output.
Obviously, letting the magic smoke out is not uncommon, but I don't think I've ever done this much damage this efficiently before... Share your stories for mutual schadenfreude!
Edit: Well it looks like the pros have put a lot more money up in smoke than me... In other news, having had a night's sleep and a day at (non-electrtonic) work, I've returned to discover that one of these boards failed as a dead short and protected the rest from dying. Lesson learned, I think.