About 10 years ago I was getting ready for the season opener on elk here in Colorado, and two days before season I discovered I didn't have any elk rounds loaded for my .270. I was busy with work and family life, besides getting all my gear together, so the only free time I had was after everyone else had gone to bed. I already had a proven load that I used, so I thought it would be a simple matter to load up 20 rounds. Well, with my mind on other things and being fatigued from racing around, I picked up a can of IMR4198 instead of IMR4831 out of my reloading cabinet unknowingly. I went to the range the next day during lunch to make sure the rifle was still sighted in, and on the first shot things went to H*** in a handbasket. By the way, 4831 is a slow burning rifle powder, 4198 is at the other end of the spectrum, pretty fast. My first clue something was wrong was the really excessive recoil, and powder burns on my trigger hand and face. And it was really loud. Fortunately the rifle, a Remington 700, held together. The case had completely ruptured, the front half of which went out the barrel, the back half flowed around the locking lugs and also into the firing pin hole. Most of the gas went out the vent on the side of the receiver, and that which came in the firing pin hole was deflected by the shroud on the back of the bolt. Yes, I was wearing shooting glasses and hearing protection. The bolt was completely locked up, and the bolt handle broke off while I was trying to get it open - no big deal as the bolt and components were toast for the most part anyway. I sent the rifle into Remington for repairs, and they fixed it for free. It turns out that I had the old style 2 position safety, and Remington included a new bolt with the new safety upgrade.
Anyway, when I got home from the rifle range that day, I went straight to the reloading bench and discovered what I had done. I guess the lesson here is that you need to be all there when reloading - awake and undistracted. And don't forget to wear hearing and eye protection when shooting. So far, that is the only major mistake I have ever made in 35 years of reloading. I seem to remember getting a few primers in upside down on a couple of rounds, and not getting enough lube on some cases and getting them stuck in the sizer die, but that's about it.