Grief - so many possible contenders I think I might share an almost a mishap, instead.
I've worked in some pretty bizarre settings over the years, but a couple of years ago I got asked to advance the electrical install for a big party the RAF were having in one of their hangars at a station. Trouble is, in some piece of genius planning they hadn't cancelled what the hangar was used for early enough - so to cut a story short, I'm up really high in the rafters of this reinforced hangar (think they call them a HAS?) whilst there is a taxiing, armed to the teeth with genuine weapons about to take off, Harrier UNDERNEATH me, on the Cherry Picker. I got a quick whoosh of fairly hot engine exhaust, fingers got really sweaty and slippery really quickly, and I dropped a 3T shackle pin (weighs about 500g, solid steel bolt). Amazingly, it missed ÂŁ45m of Her Majesty's property by a nats whisker and no-one noticed.