Incorrect weapon damage. When I can shoot you point blank in the face with a shot gun slug and you kill me, but I come back and kill you with a BB gun in three shots to the knee?
The term is "critical existence failure." It happens a lot in video games. You can get dropped from 100% to 1% health in one huge hit, and your character doesn't even flinch. But the next bee-sting that hits for 1 point will kill you.
The thing that makes me fume the most about this : fallout has a system in place to make this much better (maybe not quite perfect, but better) Limb Crippling. A crippled leg should be 10x worse than it is right now. Make the player move at encumbered pace, and slowly lose hp (bleeding) until the crippled leg is fixed.
Likewise with arms. Crippled arms could reduce your encumbrance by 25% (hard to carry things with broken arms) in addition to the aiming penalties.
And the head. That's gotta be the lamest penalty ever. Sure, the blurry screen is annoying, but I'm fairly certain getting your noggin crippled would have way more effect. If nothing else, crippled head should instantly drop fatigue to 0, like boxing gloves or the cattle prod.
Oh, and these should be for both player and NPC. If I cripple my opponents head, that fool needs to fall down.