I absolutely 100 % agree. But I'm not sure that I agree in the exact way that you intended it, so maybe you can confirm either way.
Unforeseen consequences as an outgrowth of action resolution is one of the beating hearts of keeping conflicts dynamic and interesting. Here are all the moving parts:
1) The consequences (presuming failure here) needs to address what the thematic stakes were about in the conflict. Do you recall a long, long time ago when we (I'm almost certain you were involved in that conversation) my 4e play excerpt where the PCs were on horseback sprinting across the badlands trying to get to the forest to lose the army of bad guys on their tail (after they just stole an idol from their temple to bring back to the forest's Shaman to lift a curse)? They failed a navigation check (it was Nature if I recall) and it was the 2nd failure of their Skill Challenge to "escape the pursuit by making it to the forest." I navigated them getting lost and cresting a rise and narrowly stopping their horses before falling into a large gorge (with the forest in view on the other side).
Offhand I don't remember this example, sorry. But I get the gist.
"Unforeseen consequences" that set them back in their goal and created a new obstacle to overcome (as the scene's conflict mechanics said things were still in the balance).
From the players' point of view the consequence of the gorge is unforeseen. It matters not whether the GM had the gorge on her map all along or made it up on the fly (in badlands a sudden gorge makes perfect sense either way).
2) Unforeseen should mean all participants.
No, just the players. Ideally the GM has already thought of a bunch of possible outcomes and thus won't be caught off guard.
The more the GM contrives to preconceive a outcomes, the following happens:
a) The GM's precious, prepared material will have a tendency to limit the dynamism of play. There is situational context and ebb and flow and momentum and player intent that will emerge during play that will not be regarded in the GM's preconception of events before play ever began.
b) The GM won't get to "play to find out."
c) The game will be increasingly apt to be seduced toward GM Force in any singular moment of play and possibly have a tendency toward erecting a railroad for the long haul.
First off, there's a difference between a GM having a preconceived outcome and directing play towards it and a GM having a bunch of possible outcomes in mind (or in notes) and putting these in play as the situation suggests. That said:
a) sounds like something
@pemerton, who IMO has a rather strong and consistent anti-GM bias, would post.
b) if the GM's only just now finding out what's going on, she's floundering. The GM should IMO always be a few steps ahead. In your chase example this would include having a half-decent map of the area done ahead of time so I could see what was where, and track the PCs' progress. (and the PCs would probably have learned some of what was where on their initial trip from the forest to the temple, if one was made, though when hotly pursued later they could still get lost as hell and find a gorge they didn't expect)
That's what prepping more than you need is for: reducing the chance of having to hit player-thrown curveballs and-or having to wing it, which IME often (as in, always!) leads to consistency issues when I don't remember some relevant detail I said an hour ago, can't write and talk at the same time, and don't want to grind everything to a halt every two minutes while I make notes on what I just said. Not saying I can't wing it, but I prefer not to* if possible.
c) I don't hold the same strident objection to GM Force that some here seem to. It has its place, particularly on nights (and they do happen) when the players are in story-consumption mode. Even a full-on railroad has its place now and then, though I prefer to keep these occasions to a minimum.
* - the exception is something like a dream or alternate-reality scenario or adventure where consistency doesn't necessarily matter anyway. I'm happy to wing those.
