The recalcitrant stance I'm taking in this thread is with respect to the OP's and others in the thread apparant desire to label this as some sort of artful DMing style that should be emulated and encouraged in others. I think it should be on a relatively high shelf and carefully labelled, "Use with caution.", and not at all part of the DM's regular tool set.
Let me tell you what's really going on here.
Celebrim, I have a couple of questions.
Have you ever done anything like this?
Have you never had a moment when a player said some awesome thing that would be great in the game?
Rob Heinsoo - Chumming the Dungeon
How often do you get to do this. How often do GM's zing off in the direction mentioned by the players during the game.
Celebrim: To what extent do you plan EVERYTHING in your campaign world?
Do you really have every NPCs motives planned out, in detail...
along with their tastes, their favourite colour, their personal code of honour, the names of all their relatives, etc. BEFORE you introduce them?
Do you have the complete contents of every room detailed (ie. you always know if there's a pen, a paperclip, or a stapler, in any given room in a modern setting)?
Because you seem to expect others to do so, with your dismissive attitude towards winging things. But, see, that takes a lot of preparation. And for many GMs, there's no fun to be had in that extra preparation.
Whereas, many GMs can deadpan "he has a fondness for fish" based on the fact that the person started buying fish, and, well, someone's going to have a fondness for fish surely?
In fact, if you took that twist mentioned and used it in another adventure with the same players then you have done it.
The fish event, did you have the NPC already written as a connoisseur of fish? If not then you've done it, right there.
Would that have broken the trust? Are you saying that it would have been a bad thing? Would it have broken the game?
Don't get me wrong. I'm absolutely not dismissing your DM style. Not one bit. What I'm failing to see eye to eye with is the sense that you seem to say that others only use such a thing to paper over some mistake or that it is somehow, even used lightly or just once, a break of the trust between DM and players.
Which isn't to say that you can't invent things in midstream. In my current campaign I inadvertantly created a new red herring improvising a scene I hadn't anticipated, and it's a good one and some of the PC's are already lured by it. I briefly considered completely changing who the main villain is. However, I realized that while its a good idea with lots of play potential, it's also a sterotypical one and would be completely unsurprising. I'm not going to change the story to put it on the new path, but I probably will develop contingencies along that path in case the players end up chasing the story in that direction.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.