I'm the kind of guy who has pretty limited skills with tools and I'm no kind of craftsman. In working with wood, for example, I can't really build anything. I van follow directions, however, so when working on my cars and cycles, I stick to only the things I know I can handle.
It's knowing my limitations that has served me so well at the load bench. I follow a lot of rules and take a lot of care to build good loads. I suppose because of that, I've been very fortunate in more than 20 years at the bench.
I once made a hundred rounds of .357 Magnum with cast DEWC bullets and ran them way
too warm. Not like too much pressure warm, but like too much speed to the point where a dozen of them leaded the hell out of my barrel. This taught me three things -- one was that you can't hot rod some cast bullets without paying a price. Two was that you shouldn't ever make a hundred (or more!) of anything until you know it's proven success. And thirdly, when you put a serious roll crimp over full wadcutter bullets, it's complete hell trying to break them down and un-do what you've done. I ended up cutting the rounds apart to break them down. I saved the primers but lost the brass.
Probably the biggest error I have ever made at the bench was loading too wimpy. I was using a powder that meters really poorly and I started with a load that was just too conservative. With a plated bullet and a too-light load of Green Dot, and a too-long COAL, I stuck one in the bore of my favorite revolver. And it was awful work getting that stuck bullet out of the bore. In retrospect, it was just plain ignorant to be so conservative with a powder that doesn't meter well and even more so when loading a new load in .38 Special and launching it from a .357 Magnum. There was no logical reason to start as crazy light as I did.
So in many years and many thousands of rounds... those have been the biggest errors I can remember.