This hapend few times already. When I shot an AI enemty, some times you can shot a whole magasine and he won't die.. He will just keep walking but dose not shot you anymore and after a while he will suddently fall on the ground.
This thing ususualy happens on Multi Player games, like lagging. You shot a guy he don't die on your screen and see him moveing but in real he is dead. {F16999}