If you design your own worlds and the games in them, I reckon this is superb mental exercise for both the DM and the players. It really does teach everybody involved to think both outside the box and on their feet. My aim as DM is always to try and present traps, puzzles, and problems that the players haven't met before. For instance, in every game I have ever run, I always include at least one scenario in which the players have to manoeuvre so that two different (and extremely tough) opponents knock each other out. And if they get it wrong, like trying to take out each of the opponents in straightforward fashion, the party is likely to get very badly hurt. Ever seen four level 4 characters trying to take out an iron golem in simple combat? Nasty ...However, I find that no matter how careful I am in designing the game and the world, a good set of players will come up with things I haven't thought of. When that happens, I have to come up with answers that are (i) not illogical, (ii) fit in with the world I have designed, and (iii) do not invoke a "deus ex machina".
For example, a DM should never put himself in the position of having to forbid the use of a specific spell in a specific situation without prior warning. If the players are good enough to think of a way to "short-cut" a scenario, then they should get the credit and the DM must live with it - and design his scenarios more carefully next time.
That's what I understood you to mean! 