Noticed several times when attacking creatures, thier hit points say they have no hit points but it still takes two or three more shots to kill them. I have had this happen randomly on different creatures. This was the first time I was able to catch it on a screen shot. For this one in the screeen capture I switched over to the shot gun and it took two hits before I was finally able to kill it.
http://i635.photobucket.com/albums/uu71/searoverx/ScreenShot40.jpg