Becoming complacent and overconfident with tried-and-true procedure rather than proceeding with all precautions will lead to absent-mindedness and mistakes. I have to constantly remind myself to triple-check myself at work with all aspects of operating CNC machinery. We do smaller runs rather than manufacturing quantities so we see a large assortment of job/parts come through and each gives prime opportunities for mistakes.
One misplaced digit in coding or a simple offset can set off an alarm if you're lucky, or cause many tens of thousands of dollars in damage to machinery and machine downtime/repairs if safety check procedures are truncated and abridged in practice. I've seen others at work crash machines and it's because people try multi-tasking which divides their attention, trying to rush things through, or become too complacent and make assumptions. Logging 50-60 hours a week on stuff like that for 48 weeks out of the year, you're going to eventually going to slip up given a long enough time statistically, but being cautious and having safety checks minimizes the chances of such if you follow them religiously.
I think it's even more important with firearms because your life can be at stake. Take your time and avoid distractions. As with anything, if you fiddle with guns long enough, you'll have an "accident/negligent discharge" eventually due to events within or outside your control, wether you're a casual weekend shooter or some super elite ex-military guy with tons of trigger time. However by taking all necessary precautions possible, you'll greatly reduce the chances of those...perhaps even to the point where you won't see one in your lifetime.
