Essence at [unixtime wrote:1111819671[/unixtime]]
If the players try to accomplish a goal, and you force them to fail because they didn't do some arbitrary thing that you decided on beforehand, you are being a spoonty DM. If the players don't try to accomplish a goal, and you force them to accomplish it against their will, you are being a spoonty DM. The players should be given a chance to tell their character's stories. They shouldn't fail to do so, and they shouldn't be telling the DM's.
I'm not quite sure I understand you... you're saying that basically any mystery plot should be pretty much autosolved by the PCs regardless of if they make good choices searching or bad choices searching?
Where's the fun or challenge in that? I don't really think I'd want to be in a campaign that lets you succeed all the time just because you give effort. That's like the DM fudging dice in combat so that PCs never die in battle. It just cheapens the game experience. Now, inevitably some people can come back and say "you may never know the DM is fudging" and all sorts of other defenses toward fudging the dice or the story, but all in all sooner or later you get some indications.
Within every quest there should be an option to fail, and that option should lie in more than just bad dice rolling. If the PCs make poor decisions and fail, then that's fine, and I'm not sure why it's bad DMing to just let them fail.
It's going to get pretty stupid if every locale the PCs decide to check turns up a lead, even when that lead seemingly has no good reason for being there. To me, a DM doing this is saying to the PCs, "OK you guys are too damn stupid to figure out the quest I had planned, so now I'll just railroad you to the solution so I can advance my plot."
If you want to let PCs tell their own stories, you
must let them fail sometimes. Sometimes the princess doesn't get saved, sometimes the mystery assassin remains undetected. If you want to get away from railroading part of the plan has to be to not have a predesired destination you force the PCs to reach. If they can't solve the mystery, then they can't solve the mystery and you move on with the story.
That feels a heck of a lot more real than throwing some timely clue that always happens to appear in thier path when their detective work is sucking.
Part of your planning for a mystery quest should be a contingency in case the PCs don't solve it. If you assume they're going to solve it and will keep throwing clues at them until they do, then that's railroading, plain and simple. It's like fudging dice only potentially worse. Writing off really bad luck is fine sometimes, but when the PCs decisions themselves become meaningless... that really blows.