Some years ago, I was standing duty in the armory and one of the guys who normally worked in there (I was only there once or twice a week, being normally out at the range) asked for help with an M14. There was a bore obstruction, he said, and he didn't know what to do with it. Bear in mind that this individual was notorious for being a few cards short of a deck; had I considered that first, the following would not have happened.
Bolt locked to the rear, weapon checked clear, look down the muzzle - yep, there's an obstruction. I try tapping it out with a cleaning rod. Then I try hammering it out. I flood the bore with CLP and try some more. About an hour into the evolution, I've now braced the end of a (shortened) cleaning rod against the concrete floor and am hammering the rifle down onto it with both hands. At this point, I realize that something is seriously wrong.
"Dude," I say, breathing hard and sweating, "What the hell happened to this thing? Who was the last person to check out this weapon?"
"Oh, the drill team," he said helpfully. "It's a ceremonial weapon." A ceremonial weapon...as in, demilled. Whatever the bore was filled with, it wasn't meant to come out.
"Why did you not tell me that an hour ago?"
"Oh...I probably should have, huh?"
I nearly threw the rifle at him. I
did throw the ruined cleaning rod at him. And every time from then on that he asked me for help with something, I spent at least a few minutes interrogating him before assisting. Several other potential shenanigans were averted later in this way.
My first time charging a Mk19 grenade launcher, I had been told, but did not quite understand, just how much force was required. I did not brace a foot or anything like that; after all, it couldn't be any harder than the M2HB, could it? (For those who do not know, it is.
Much harder. There are two handles for a reason, it seems.) I grabbed both charging handles, leaned back just a bit, and reefed on them with all my might. The handles did not move, but I did - right into the back of the weapon, at fairly high speed, with my groin leading the charge. I braced my foot after that.
Operating the Mossberg 500 101: "There is only one way to reliably eff one of these up," I'd tell my students. "If you load a shotshell into the magazine tube backwards, the gun will be OOC and tools will be required to get it operating again. DO NOT DO THIS!"
And a student would occasionally do it anyway. Many of these were fairly new shooters, so it was only to be expected. It was not expected for me to do it with a dummy round in front of a class, so imagine the surprise of my fellow instructors when I did just that.
I never made that mistake again and was able to demonstrate to the class how to correct the error with a Leatherman, so perhaps they got some use out of it. They certainly seemed to enjoy the ribbing that I got from my colleagues.
Not my personal screwup, but spectacular enough to mention: you know how they say not to charge a M2HB when the backplate has been removed? A dislodged recoil spring guide rod, under pressure, will embed itself in drywall ten feet away. I'm told that it can cause serious injury if a person is in the way. Never do this, but trust me when I say that it's impressive.
More recently, I was demonstrating an FAL to a potential buyer and it wouldn't cycle. I hadn't shot it in years and had no idea what was wrong with it, so I promised to take it home and look at it. The gas piston was in backwards.
There was even a slight discolored crater where the gas had been impacting against solid steel. It worked a lot better after I turned it around.
That's probably enough screwups for now. I could write a book filled with these.