I've been looking at this post for a couple days now - trying to think of my worst mistake. Not to be smug, but I've never done anything serious.
Once, I accidentally mixed about 20 SP magnum primers with about 30 standard ones. No big deal. I was loading target level rounds - I just proceeded. The difference was transparent when shooting.
I've pinched the tip of my thumb in the press when going too fast during the case flairing process - nothing serious - just enough to make me slow down.
And I once used my press to do something automotive related (can't even remember what) and damaged the ram - that was stupid. I wrote RCBS (pre interweb days) and told them I was a dummy for using the press "in a manner for which it was not designed" - or something close to that. Please send me a new ram along with the bill and I will return a check. I got a new ram a few days later - no charge.
Nothing else that I can remember. My loading process is very deliberate and regimented. Basically, I've taught myself good safety habits. I don't cut corners. I've managed to stay out of trouble. I wonder how I'll do as senility sets in