Not motorcycle-related, but I would like to enter the "dumb" contest.
When I was in the Army, we had a helicopter broken down with a bad engine starter about an hour's flight from Ft. Lewis, WA.
We were dispatched with a replacement starter. When we arrived the crew of the other aircraft was doing something else, so I removed the starter and then promptly dropped it about 10 feet onto the pavement below, shattering it.
That was when I was told I had removed the wrong starter; the good one. So, we were still a starter short.
Had to fly back to Lewis and get another starter, than back to Yakima to get 2 good starters on the aircraft.
So, figuring 2 unplanned flight hours at around $2000/hr apiece and 2 starters at around $1500 apiece (the good one I dropped and the one that unnecessarily had to go on the unaffected engine) that little debacle cost you US taxpayers about $7,000 for no good reason.
I personify Your Tax Dollars At Work.
That might not be the truly "dumbest" thing I've ever done because they've all kind of blended together over the years and specific memories are hard to pick out, but it was definitely the most expensive.
Probably for sheer stupidity, the crown would have to go to a time right out of high school when another guy and I were horsing around with one of those Buck 8" hunting knives with the black plastic handle. It got dropped on the ground. We both reached for it. The other guy grabbed the handle. I grabbed the blade. We both pulled.
I got about 10 stitches out of that one. I would like to say I was drunk, but I was as sober as could be. What the hell I was thinking when I did that I'll never know. Hell, I don't even want to know.....