I answered a Quora question a few weeks ago on What are the plot devices you would like to see less of?. Although the questioner originally asked for only three plot devices per answer, I couldn't help myself and added two more. This has actually become one of my most viewed answers on Quora. Anyway, below are the plot devices that drive me up the wall (slightly edited from my Quora answer). Note that nearly all the links take you to the appropriate entry on TVTropes.org
Someone overhears only a small part of a conversation, pieces together what they think the conversation is about, and come to a conclusion wildly different from what was actually being said (I'm going to kill him tomorrow ... at basketball). Similar examples are seeing the characters do something that looked suspicious when viewed from only one particular angle or at just the right moment. These are so unlikely to occur at all in real life (most people would simply assume they overheard something out of context), and the problem could usually be resolved with a simple question that never gets asked.
This is when characters seemingly go out of their way to act stupid. The worst example of this I can think of is Dracula. *Spoiler Alert*. Even though one character had already succumbed to Dracula, and all the lead characters knew this and believed in vampires, when another character began displaying the same symptoms, it never dawned on them that maybe Dracula was working on her, too. (In fact, Dracula has so many bad horrible plot devices I could on at length on how much I disliked that book, and have - Book Review - Dracula.)
This is the tendency of so many writers to treat skeptics and scientists simply as cynics or denialists. It's especially bad in stories where in that fictional universe, evidence for the supernatural/monster/alien is all over the place, but the skeptics still refuse to believe. Perhaps the worst example of this in a story I've read is in the Left Behind series (I only got a couple books into it). After all these events that just scream Rapture and that the fundamentalists were right all along (billions of people disappearing in an instant, Israel being miraculously saved from an invasion, fire breathing prophets), all the religious skeptics go on continuing to dismiss religion out of hand for some reason (more info - Some Early Thoughts on Left Behind, More Thoughts on Left Behind After Finishing the Book, and Book Review - Tribulation Force).
Alien Invasion movies are almost universally awful if you apply any type of rational thinking to them. First, the motivation is almost always ludicrous. This is a civilization with the technology and resources for interstellar space travel. What could they possible need from Earth that wasn't more easily attained elsewhere? Even if for some reason they wanted to come to our solar system, there are all the objects in the Oort Cloud and Kuiper Belt that would provide huge amounts of water, metals, or minerals, without the cost of removing them from Earth's gravity well. And when they do actually attack, in so many action movies, it's like the aliens have no concept of strategy or tactics. They send in a bunch of small fighters or foot soldiers to shoot up civilians (e.g. The Avengers or Cowboys vs. Aliens), when they could just drop bombs from orbit without ever exposing themselves to our military. Or, considering their level of technology, they'd probably have weapons even more effective than plain old bombs that they could utilize. It's just ludicrous to imagine that their invasion strategy would be to send a bunch of their alien soldiers into Manhattan.
Santa Claus Movies Where Kids Should 'Just Believe'
These movies irritate me to no end. In fact, I've written about it this blog before in the entry, Yes, Virginia, There Are Liars. Why do so many movies make it a virtue to accept something on blind faith without evidence, when we should be teaching our children critical thinking skills. Skepticism is what keeps people from buying timeshares, giving their credit card numbers to Nigerian princesses, or believing they've won the Internet lottery. It's a skill that should be fostered, not made to seem like a character flaw. And the Santa Claus movies are especially irritating because every sane adult knows the truth about Santa. We're not just telling kids to have faith, we're telling them to have faith in a known lie.
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