Vampire
Failing to manage hunger can cause Coen to lose control, which can kill people and remove the quest content attached to them. The walkthrough states the rule cleanly, and the conditional in it is the whole mechanic: **unless you have lost control to hunger, you choose how much to drink.** Stop early and the victim goes down unconscious; go all the way and they die. Control is the default and hunger is what takes it away, which inverts how this usually reads — the risk is not that a button is mis-pressed, it is that the choice was forfeited several scenes earlier by not feeding. If you would rather not touch people at all, animals and some other creatures are a standing alternative. Rebel Wolves also warn about the specific failure this exists to prevent: draining someone important, with consequences they describe as unpredictable.
The rule is stated on camera; the thresholds behind it are not. How hungry is too hungry has no published number, so a no-kill run cannot yet be planned to a margin.