Familiarity, training and complacency.
I spoke with a shooting bud earlier who had a close call - he ND'd when prep'ing his carry piece for routine clean. No prob's - rule #2 thank heaven saved the day - just a small ''scar'' on the concrete of his basement (and a red face).
He is a shooter of some 20 or more years and has carried for many years. He shoots IDPA and trains quite regularly.
I asked him - how come? His response was that he ''just forgot''!!! He had dropped the mag on his G19 but - yep - there was a chambered round!!! Oops.
On discussing the event it became clear that in a sense, the longer we go on, and the more ''skilled'' and familiar we get ......... the more also we may potentially become complacent. So yeah - ''done this 100's of times, no sweat''..... bang! Oops.
Bottom line conclusion we reached was simple - all gun handling still requires conscious thought and not just the ''yawn - done this before'' approach.
I have gotten close to having done similar but fortunately have tried to take a step back and run thru mentally what needs done first ......... it is only a heartbeat's distance from trouble, if we let familiarity and supposed training, mandate our casual actions.
Stay safe folks - it is a good way to go.