Prime example of this happened to me this weekend, 2x though not with handguns.
Example #1:
Just doing some target shooting with my Marlin 39a. I rarely if ever shoot it, and I loaded the gun and pulled the trigger 2x, cycling the lever both times and looking in confusion at the live rounds that were ejecting out on the ground.... Then I realized I left the stupid crossbolt safety on.
Right afterwards, I shot my M&P 22-15 which I shoot all the time. I naturally flick the safety off every time I look down the sights.
Example #2:
Later that afternoon, while crow hunting with my brother. There was one swooping in low right out front in my "zone" and I pulled up getting a click instead of a bang. My brother saw it and was expecting it to be his bird, but I cycled out the "dud" round and had another in the chamber before the bird even started to turn and nailed it. (Benelli click, wasn't fully in battery for some reason
)
Example #3:
On a later stand, they were coming in hot and heavy. I emptied my gun but another one was coming in fast and my brother said that in the time he had to raise his gun and take the shot, I already had mine loaded +1 and ready to take the shot.
I only use Benelli shotguns so my hands go through the motions even though my mind may be occupied on watching that crow making his last dive.