Returning again to [MENTION=6696971]Manbearcat[/MENTION]'s Mt Pudding example: the goal is to get to the top of Mt Pudding and find the pudding. Without the divining rod, that goal is no longer automatically achieved simply by getting to the top of the mountain. So when the failed climbing check is adjudicated as "You lose your diving rod down the crevasse as you narrowly avoid going into it yourself," the character has not achieved his/her goal, and in fact has become less likely to achieve it.
To give another example from my BW game:
The PCs were in the Bright Desert, south of the Abor-Alz. The party had become separated: the elven princess had been captured by orcs, the sorcerer assassin had run away from the same orcs, and the princess's retainer and the mage were still at camp waiting for the others to return.
The sorcerer resolves: to rescue the princess from the orcs I need an army of tribesman, and (to quote the player) in these parts Ancient Suel tribesmen are as thick as fleas on a dog! So the player checks his circles, adding in his relevant bonus dice derived from his reputation as a minor illusionist among the outcasts and wanderers of the wastelands. The check is a failure, and so one of the tribesmen he once knew - Wassal - captures him and the retainer as they trek through the desert sending out calls to the tribesmen. Wassal is hostile to the mage, because he blames him for bringing orcs into the desert. (In my mind, this was very loosely inspired by Conan's unanticipated reunion with the tribesmen in "The People of the Black Circle", except with less warmth and more hostility.)
Additional complications ensue, as the mage tries to explain that it his brother Joachim, possessed by a balrog, who is responsible for orcs coming into the desert (the relationship between the mage PC and Joachim, and the mage's quest to end the threat that his brother poses to the world, is one of the key driving elements in the campaign). Further complications ensue when Wassal discovers that the mage is carrying a spellbook inscribed by Joachim, and the PCs are kicked out of the tribesmen's camp into the desert.
The PCs eventually do catch up to the orcs and rescue the princess (I think with a successful Tracking check from someone), but without any help from the tribesmen this takes longer than it otherwise would, meaning that the princess suffers a relatively serious injury with lasting debilitating consequences, and her retainer is shot in the chest by an orc and barely survives.
Being injured in the course of fighting the orcs, the PCs are also unable to push on into the desert to find the pyramid the orcs (whose party included a serious siege engineer) were heading towards. Instead they have to rest for three months until the retainer is well enough to be moved, and then head to the ruined tower in the Abor-Alz.
That's just an example of the sort of play that "fail forward" tends to generate. The PCs aren't guaranteed to achieve their goal (and, in this case, they ultimately didn't). But there is a guarantee of narrative momentum.