Dice fudging is the most unforgivable action a GM can make.
If the DM want something to happen, because of the story, because the player did something creative or reasonable, because it would be better and etc, he should just say: "That happens." No need to roll unless you're ready to keep the thing going on the direction the dice says it goes.
Dice fudging is bad GMing. Any game that encourages it or even rolling dice behind the screen is BAD.
All dice rolls should be OPEN for all to see and thrill in the anticipation, pondering about the possible outcomes that fate will decide.