Since I'm in the process of plotting a book, I thought it would be a good time to discuss some plotting pitfalls (mainly as a reminder to myself!). One big pitfall is what I call "Idiot Plotting." That's when your characters have to act like idiots in order for your plot to work -- they have to abandon all common sense, forget their training and experience, and otherwise do things no person with more than two brain cells would ever do. Otherwise, you'd have no story because they'd solve the puzzle or stop the bad guy in the first chapter, or they'd never get into the jeopardy required for a climactic scene.
In horror movies, this is when people go into the dark basement alone. In amateur sleuth mysteries, this is when the protagonist goes alone into the villain's lair without telling anyone her suspicions about the villain's identity. In fairy tales, this is when the heroine does the one thing she's been explicitly told not to do. In romances, this is the Dreaded Misunderstanding, in which the characters don't have the one simple conversation that could resolve everything and are willing to throw away their relationship rather than have this one conversation.
And yet, you'd have a pretty boring story if everyone always did the smart thing and always made the right decisions. How can you keep the plot going without making your characters into idiots? Here are a few suggestions:
1) Withhold information from the characters -- they can only make good decisions based on the information they have available, so make it harder for them to get the right information. Make sure they're still trying to get information, but discovering clues is a good way to throw in plot twists that make them realize they made bad decisions earlier. This gets tricky if the audience is in the know while the characters still aren't because it's easy for readers to forget that the characters don't know everything they know.
2) Let the reasonable precautions fail -- Let the characters do the smart thing and try to prepare themselves, only to be stymied by factors out of their control. The lights are actually on in the basement, but the villain cuts the power once the hero is down there. The amateur sleuth calls the police officer, but he's caught up in another crisis on his way to join her at the villain's lair. The batteries in the phone or flashlight fail at a bad time (goodness knows, this happens enough in real life. Just don't overuse it). Someone else swiped the thing the character was counting on being there in a crisis. The person who was supposed to stall the villain fails in the task.
3) Provide adequate motivation -- One plotting exercise I've seen recommended is to think of something your character would never, ever do, and then try to come up with a reason to force that character to do it. That's a good way to avoid an Idiot Plot. Have there be stakes that explain the bad decision. The sleuth has to go into the villain's lair NOW because there's someone in immediate jeopardy. There's actual evidence for the Dreaded Misunderstanding, and it ties into something in the character's background or experience. Doing the one thing that's forbidden is the one way to save someone else. Make it clear that the character has no other choice but to do this thing, regardless of how stupid or dangerous it might be. The readers should feel like they'd have done the same thing in those circumstances. The more drastic actions require more drastic motivation.
4) Make the villain smarter -- You can avoid a lot of Idiot Plotting by creating a villainous scheme that isn't so easily foiled. That way, the heroes can be smart and still fail. The villain can plan for the things the heroes are most likely to try. After all, Idiot Plotting applies to the villain, too. You've got something of an Idiot Villain scheme if the heroes could foil it in chapter one by just being reasonably intelligent. This is when I generally refer people to the infamous Evil Overlord List (http://www.eviloverlord.com/lists/overlord.html). One big problem I see in Idiot Plots is the villain acting like a villain and the heroes not noticing. If the villain is subtle and reasonable, then it's okay if it takes the heroes a while to figure out the villain's identity or to unravel the evil scheme. But if the villain is mwa-ha-haing all over the place, then the heroes need to at least suspect something. If the villain is gloating about his evil scheme, the heroes should be proactive and have a head start on stopping it instead of being caught by surprise at the last second.
5) Set it up properly -- if you've established through the rest of the plot that the hero has a weakness or blind spot, that his technology is often unreliable, that his sidekick panics in a crisis, that his cop ally is seriously overworked, then you can get away with making things go wrong in order to get your hero into jeopardy for the climactic scene. The trick there is to not make it something the hero reasonably should have corrected -- if his phone has been cutting out off and on through the whole story, then there needs to be a good reason he hasn't paused to get a new one that works. If he knows his friend is unreliable in a crisis, there needs to be a good reason he resorts to depending on that friend. Using the hero's weakness or blind spot works best if the hero is conscious of the weakness and deliberately trying not to play into it -- and that's the one time when the weakness would have helped him avoid trouble. The weakness or blind spot doesn't work as well when you feel like the hero should have learned from previous mistakes.
In general, avoid any plot that has readers shouting "you idiots!" at the heroes.