"Sometimes you have to roll the hard six" ~Admiral Adama, Battlestar Galactica
Do you know what that means? It's a term from Craps, and essentially means rolling a pair of 3s on two six sided dice, instead of getting a six a different way. Rolling a hard six pays 7-1, as opposed to the 7-6 that any other six gets and there's only about a 3% chance of doing that. (Facts from urban dictionary, so not verified or anything. But it works). Essentially, any time you need to roll the hard six, you are saying that it is a low probability shot, but you need it to come up. In game terms, think of it as the clutch nat 20. The crit that comes just at the right time, and saves the day for everyone. Its a great feeling when you hit it, but with a 1 in 20 shot you only have a 5% chance of hitting it. Think about that. A 5% chance of pulling off something epic and amazing at just the right time. Probably actually lower than that, especially because it needs to be at just the right time.
In Game Design, I've heard other designers refer to this phenomenon as the 'Whiff Factor'. I heard it first from John Wick in one of his video blogs, but I've heard it in enough other places that I've started using the term too. Now, the whiff factor is different than just the chance of rolling a nat 20 at the right time. It has more to do with the drama of the situation, and is one of those things that is best explained with examples first. So let me use one of the more easily understood ones.
You're playing in a campaign, the game has been going on for nearly 2 years and throughout that entire time your character has been hunting the man who killed his sister. Finally you've found him. He is in the same city, you are trained and ready. You've become powerful. At long last your character arc can come to a proper end, and the moment you've been anticipating for 2 years of real time, perhaps more in game, is about to come up. You follow the man into an alley, and draw your sword. You have him cornered, you explain why he's going to die, and you attack. whiff. You miss and he gets away, or kills you.
Do you see what I am getting at? The dice, despite what everyone may want to have happen narratively come down the other way. What should be your character's moment of closure, is suddenly a moment of loss and defeat. Not necessarily a bad thing, but it can be very frustrating.
As a game designer, when making your system, you need to decide where your game is going to come down on this though. Do you leave it all up to the dice? Let them be the final and ultimate arbiter, and if a GM wants to change it they'll have to do it on their own? Or do you build something into the system to help deal with the Whiff Factor and give the PCs more control over what can, and does, happen in the game?
Neither way is wrong, and they both fit different play styles. Some players really like the idea that they can fail at any moment, even the important ones. They like the challenge, the finality of the dice roll. The fact that it makes it more like real life, and not a game or story. A world where they can lose, and they can lose in an embarrassing and final way. Others prefer it to be more like a story with them as the heroes. They don't want to spend 2 years working towards something, only to have it end in failure despite all their efforts to the contrary.
Depending on the game, the setting, I like both. However, I tend to fall more towards the 'story' side of things than the dice as final arbiter of everything. I like the players having some way to mitigate the whiff factor, to achieve what is truly important to them despite the dice. However, they do still have to pay for it. They can avenge their father, but there is a cost to it. They die as well, or they have the wrong person. They lose everything else because of their quest for revenge. Something must be paid in order to over-rule the agreed upon decision maker.
But like I said, everyone's mileage on this may vary. So, where do you come down on it?