Okay, picture this scenario:
[sblock=Spoiler for what may actually be a fun plot]The PCs are college students belonging to PETA (people for the ethical treatment of animals.) They are breaking into a lab known to use animals as test subjects. What the PCs don't know is that it's a secret government lab, and the test animals are now very powerful psionics who have managed to control the humans and who have killed the scientists, one by one, for the animals' sadistic amusement.
Unknowingly, the PCs walk down the hallway when a mind-controlled lab tech pops around the corner with a fire axe. She is babbling and unskilled and has clawed out one of her own eyes. She is clearly dangerous and insane; the PCs want to disarm her while she's still talking and before she attacks. "Pull to grab the axe away from her," I say.
Well, there are four possibilities on the pull.
1. They pull and succeed. That means that they're successful in what they were trying to do. In this example, they've managed to yank the axe away from the lab tech without getting significantly hurt. I'd probably have them pull at least one more time to avoid getting bitten or clawed by her in the tussle.
2. They elect not to pull, or they start to and change their mind. In this case they will fail in whatever they're trying to do. In our example they try for the axe and don't pull it away. (Sucks for them, because I'd then have them pull to avoid the inevitable axe blow. If they didn't want to pull for that, either, they'd be struck by the axe. It would hurt them but not kill them, since normally only knocking down the tower does that.)
3. They take one for the team and purposefully knock down the tower, killing themselves but succeeding in their intended action. They would be killed by the axe but in a dying act of self-preservation, rip it from the hands of the mad-woman.
4. They pull and accidentally knock down the tower. Their action failed. Either immediately or soon, something will remove them from the game. In this case, the axe catches them in the throat. They'd turn gracefully and spray hot arterial blood across all of their friends' faces.
But what if I didn't want to off them so quickly? Perhaps I'd have one of the animals mind control them; I'd take the player aside, explain that they were now an agent of the animals, and have them secretly lead the rest of the PCs into real danger. Or maybe I'd use the "dead man walking" rule, where they'd take the role of "that guy everyone knows in gonna get killed" in horror movies. Their death is inevitable, and it's only a question of when... but they can still play a while longer. Either way, though, once they knock down the tower they are no longer permitted to pull from it, and anything difficult they try to do will automatically fail.
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These alternatives work nicely; after all, if someone knocks over the tower when trying to notice something, they can't get killed by a spot check! So you kill them in a nifty way soon afterwards, or do whatever is the most fun / horrific / impressive way to remove their character from the game.
