I've always had great long distance vision. While shooting black powder muzzle loaders I started getting in the bad habit of shooting over my shooting glasses. I could see the sight picture clearly as well as the target at distance, my glasses messed me up somewhere between the rifle and the target. Peep over the glasses and everything is crystal clear. One of my rifles had a particular hooded front sight that made it even more difficult with glasses on. I shoot mostly American made or custom built rifles but do have a Lyman Great Plains rifle which is made in Italy and therefore metric threaded. I have spare nipples for all my rifles and decided to change nipples on the one that currently was giving my sight problems. I went through my kit and found a nipple, gave it a touch of anti-sieze and lightly threaded it into the drum, it went just fine...I gave it a fairly firm tug with the nipple wrench and got back into shooting. I fired probably a dozen more shots, got down behind the peep sight, peeped over my glasses and touched off the shot. Immediately I was hit in the eye with a hot blast, I jerked upright, my buddy came running over, I could see out of the eye but it was tearing up like crazy and seemed irritated. We looked at the rifle and could not find the nipple, we never did...(I laugh about it still being behind my eyeball). What had happened is I had put a metric nipple which is just off a couple though, ever time I fired it backed itself out of the threads until finally it blew. Fortunately my eye was behind the sight disk which is nearly an inch across, still the hot gas blasted through the aperature of the disk enough to scar my lens. Eyesight is very precious, I've always been careful by wearing shooting glasses but this bad habit could have been catastrophic. When I went in for a yearly eye exam the doc said I had a slight scar in the place where I was positioned, I have to work with it today, moving my eye around a bit to get a clear picture.
I've had trying days dealing with members and non members, they are usually responsible for a great day at the range turning into a job I don't get paid to do.