I have done the 16/12 mistake. Was shooting sporting clays and unloading between stations. Popped one out, missed it and it fell on the ground. Guy in front of me had the same brand shells in 16 ga and was older so decided to just leave a live ejected shell at each station instead of bending over to pick it up. Can't fault him though, I should have checked.
Had i been the guy cutting the grass and set it off with the lawnmower blade it would have been a different story.