Whilst exploring the Commonwealth, I was near Hangman's Alley when I got into a fire-fight with a bunch of Super Mutants and two Mutant Hounds. During the fight I managed to dispatch of the first Mutant Hound, however, when the second one came at me, it was immune to gunfire and melee attacks. It wasn't even target-able in V.A.T.S. Has anyone else came across this problem?

I have video evidence as proof but I'm unsure how/if you are allowed to share it on these forums.