Each day as I put on or remove my gun belt, I check the magazine in my pistol and the backup mag in its sheath. I always rap each against the heel of my hand, to make sure the rounds are seated firmly, so they'll feed properly. Then I push the exposed round back and down, just to make doubly sure.
One day, I found that the first round in the magazine in the pistol - which I'd carried all day - had moved forward about 3/16 of an inch. It likely would not have fed properly; I'd have been able to fire only the round in the chamber, followed by an failure to feed the next.
I checked that mag periodically throughout the day, and determined the spring had gone bad. I've practiced with partially filled mags, to require changing at different intervals, but ... wow. One shot? I need to change my practice routine to include that possibility.
I know I'm preaching to the choir, but check 'em, guys and gals. If you're in the wrong place at the wrong time, it's good to know you'll get that next shot.
But if you carry with a round in the chamber, be prepared for failure after the first round. If it happened to me, it can happen to you.
Have a plan.