I've mentioned this before elsewhere, but it's not the plot itself that is the problem, its a Game Master who is unable to think on their feet, who can only see the road they had originally mapped out, and can't adapt to the unexpected as things (inevitably) change.
Some people think they don't like plots- they want action, they want destruction, they want power, they want their character to kick ass and take names and they don't care about last week or next week. Ok, that's fine, and I like most of that too, but, imo, pound for pound, that same stuff wrapped up in a good plotline makes it SO much better! ymmv, and should, but at some point another night of rolling dice and dominating against big piles of NPC's in a vacuum just isn't enough for most.
In the final tally, the sole job of a Game Master is to entertain. Cartoon quality or high operatic drama, if the Players have fun, it's all good. (It's also important for the Players to entertain the GM, but that's a different discussion.)
For those players who decide they want a story wrapped around their action, then the Game Master must become a Story Teller. And, imo, the most important thing for a Story Teller to remember is that rule about being a Game Master- at the end of the Story, did people have fun?
As the story unfolds, the players don't know what was "intended" until it shows up- and then they don't care if you designed it that way, or it was sudden inspiration - if it was fun, if it was what they liked, then it was right!
A pre-designed plot, carved in stone, is likely to fail in this regard. Not because it was poorly designed, but because at some point the players will want to do something else, but the plot will demand a "ring in the nose" feeling to show up, and the players will (rightly) stop believing they are in control, and not just passengers on some fun-ride. And the fun stops.
A good Story Teller should never let too much out. And if there's a single douse roll that needs to be made, do or die, they either need to make that roll themselves (and be willing to fudge it!), or be willing to live with the failure. If they can't see that far ahead, then they need to!
Octuple botch? No problem by me- the story now becomes about the Twilight experience, and we'll get back to you later. The BBEG will still be catchable, just differently, and that evil spell they were going to cast is going to take a LOT longer than the Players had thought (for some reason.)
And a SG doesn't have to explain themselves- let the Players guess, or keep it mysterious. So long as, when the time comes, you have a good explanation, they'll trust that it's been there the whole time, because they have no reason not to.
(One of my favorite tactics is to listen to the tabletalk, and weave my plot out of the Players' paranoia and "what if's" discussions.)
"Wait, before we go to Stonehenge, there's a fairy I've been meaning to talk to..."
Feh. I am not impressed. So guess what- it turns out that fairy knows something that ties in to Stonehenge - lucky thing that Character went to see them before, huh?!!! Man, if they hadn't gone to visit that fae, they'd have been screwed!
"Your Sesneschal Odin comes through the door"
Ha, like no SG has ever been there. Keep a straight face, and either 1) stay cool, deny everything, blame it on an old head injury, or b) pretend it was a secret that he was a priest of Odin, or iii) pretend that you're messing with the Players, and creating paranoia. "All intentional, you figure it out."
But the best is to do all 3, and let them guess which. Maybe he IS a priest, and not the real guy after all! Maybe he was "possessed" by Odin, who leaves once the magi suspect. They never knew the first plan, why does it matter which is the final "reality" so long as it changes nothing? Again, why hesitate to change a decision when the Players don't know about it in the first place?
The ST drops his die and rolls a botch.
Amateur. There's a reason for GM screens. All dice that matter are secret rolls, unless I can live with both amazing success or disastrous failure. (see above.)
And double amateur- a botch doesn't equal to "failure", it means something totally unexpected happens. A huge Vis-powered Mentem botch? Maybe everyone gets brainwiped or whatever, including him. Hell, if the SG's got an imagination, he really should use it, not quit.
A short-sited SG puts the adventure down the road the Characters are traveling, and if they turn, the SG pisses and moans and takes his dice and goes home. A wise SG puts the adventure in front of the Characters, no matter which way they go, and pretends it was there the whole time. A subtle SG changes the adventure so that the Players feel they avoided the "real" one, and found this one on their own.
X happens. Was it planned that way, or spontaneous? Were they manipulated to fit the plot, or was the plot changed to fit their actions? If everyone is having fun, and no one can tell the diff, what does it matter?