Last night while playing I encountered a Giant Rad-scorpion with a health bar that would not deplete regardless of how many times I shot him, threw sticks of dynamite at him, used vats mode etc...
Thinking this one of the many bugs I've encountered I immediately reloaded my last save, only to encounter the same problem again in the exact same spot (somewhere near Good Springs cemetery).
Initially his health bar is full, but after I start shooting him it shows as empty and stays that way even though he's not taking any damage whatsoever. As you would suspect he inevitably chases me down and kills me because I CANT DO ANY DAMAGE TO HIM!!