The Ranger has a starting move of Called Shot.
When you attack a defenseless or surprised enemy at range, you can choose to deal your damage or name your target and roll + DEX.
Head. *10+: as 7-9, plus your damage. *7-9: They do nothing but stand and drool for a few moments.
Dungeon world is about following the fiction- letting the logic of the world trump a strictly mechanical sequence of events. My narration as a GM needs to make sense of what just happened. When the target is sufficiently armored and the ranger gets a 7-9, the arrow might hit the helm and glance off. Let's go with the least convenient world here. The target is a human without any helmet or headgear, and the ranger just rolled Called Shot on the head and gets a 10+, dealing 4 damage; enough to wound, but not kill. He's going to stand and drool for a few moments, but after that... what? What happens? The ranger is going to take dozens of called shots over the course of a campaign. A lot of them aren't going to be fatal according to hit points.
Options I've come up with;
Freak survival, Phineas Gage style. Works once, maybe twice, but that's going to get weird fast.
Every wears helmets or has scales nonhuman anatomy or something. Not a fan of a literal planet of hats.
Any creature with normalish anatomy that takes a headshot without protection dies. This is what I'm inclined to go with, though it seems to be making this option far better than the other two (Arm and leg) and more dangerous than intended.
Describe something else about the attack, and leave the question of how they survived an arrow to the noggin blank. Less jarring than someone getting up and acting again after I've described the shaft going in, but this feels like ignoring the fiction for the mechanics, which is bad form in DW.
How do you deal with Called Shots to the head on creatures where that really should be fatal?