If it could be easily and simply stated and codified, then we could put it into our house rules, and it wouldn't be fudging, now would it?
Let me come at the answer a little sideways. Folks who fudge say that they do so to make the game more fun. Opponents of fudging ask, "Well, how do you *know* the fudge will be more fun than what the dice roll?"
My answer is that the only things certain in human life are death and taxes. If I have to wait for certainty to take an action, I'm gong to be sitting on my thumbs most of my life. I don't need to be certain. I only need to have a certain level of confidence.
Where do you get the confidence? From experience, and from your knowledge of your game, your scenario, your players, and their expectations. With that information at hand, when it is time to fudge can be pretty bleeding obvious, even if it is hard to elucidate.
This doesn't really help me see whether fudging might be something I might ever want to do though (and it smacks rather of a high-handed illusionist approach), whereas [MENTION=9037]Elf Witch[/MENTION]'s examples were useful. One thing I got from her two cases was that the player had already been 'beaten down' by real life and in-game events, and the intervention seems a lot less arbitrary when used in such limited circumstances.
I think there's a difference between fudging in an extreme case out of sympathy towards a particular player, and a general 'I know best when to fudge' approach. The former approach potentially retains most of the challenge of the game, because players can know they'll only be cut some slack if they've *already* been put through the ringer.
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