I have missed a bird that to this day I have no idea how. Rolled him at about 35 yards. Got up to get him and off he scoots. Same gun and load that year killed one at 47 yards (mismarked my bush to shoot him at on a field) and still stoned him.
All I can say is I have had ducks on the water in easy killing range and shot them to finish them off and had the pattern explode all around them and they keep swimming. Over the head most likely if a true miss or a small nothing twig in the way that deflected enough to spare him.