http://www.youtube.com/watch?v=PlVox-G-7rg

I have had this with certain other enemies (human and creatures) where a direct hit does zero damage.

I encountered a nearly invincible scorpion in that exact spot. Boone, Ed-E and I were all shooting at this thing and the health would not got down. I ended up using melee to get the health down to the last bar and on my final hit the game froze and I lost an hour of free roaming mayhem.