Early 1990s. Loaded 300 win mag using a balance beam scale. I was going for 74 grains of H1000 behind a 220 gr SMK but missed the notch, didn't notice, and loaded 84 grains. I'm a slow learner so I got three rounds through it before I decided that something was just not right. That gun has gone through five barrels and, at the moment, needs a sixth so something like 7,000 to 10,000 rounds through thar receiver.
Shooting a benchrest match called the Crawfish in Lafayette Louisiana. Loaded my rounds for the next match, forgot the powder. Went to the line, range hot, load round, aim, ... pop. I got rounds from somewhere and shot the match but I didn't do well. Another guy did the same thing the same day - somebody loaned him a gun and rounds. He did better than I did.
Everybody who picks up range brass eventually breaks off a decapping pin by trying to decap a Berdan-primed case.
A few months ago I made up about several hundred rounds of 9mm in my Dillon 550 before noticing that the powder bar setting was too hot. They might have been okay but I was not okay with the mistake. I pulled every bullet with a Forster puller. It took me most of a day and was very very educational.
I have oversized cases, forgot to size cases, forgot primers, forgot powder, seated bullets too deep or not deep enough, used the wrong powder, too much crimp, not enough crimp, trimmed way too much, gone to the range without ammo, wrong ammo, went to a match with 100 rounds of ammo, half the right stuff and half for which I had no trajectory numbers, put the ammo box on my bumper then drove off and had it fall off on the road somewhere, no targets, no stapler, no hearing protection, loose scope base, loose scope rings, loose recoil reducer, loose stock screws, once I drove 150 miles to a range and forgot the bolt for a bolt gun. I now have a checklist, and I check it.