#3. Green Lantern -- The Guardians Forget to Take Hal's Ring Back
Green Lantern tells the story of the Green Lantern Corps, an organization of superhero intergalactic policemen who use "power rings" to defend the universe from peril. The rings allow their bearers to fly through space at incredible speeds and create any object their minds can conjure. For some reason, they decide to give one to Ryan Reynolds.
"In brightest day, in blackest night, no better actors are in my sight ..."
Reynolds' character, Hal Jordan, heroically quits his new job as a Green Lantern the instant he fails his first training exercise. Since there's no reason for him to remain in space on the Green Lantern headquarter planet of Oa, Hal zooms back home to Earth to pout and crack moody jokes about his shortcomings.
But Wait a Minute ...
Hal Jordan quit the Green Lantern Corps, but he took the ring with him. The self-appointed Guardians of the Universe let a whiny, myopic Earthling just kind of walk out of Space Police Headquarters with the most powerful weapon in the galaxy, which he then uses to rescue a bunch of people in a helicopter crash by building a giant Hot Wheels racetrack, which you may recognize as the lamest possible way to rescue someone.
"You know what? I'd rather die in the crash."
Sure, it all works out because Hal is a good guy, but what if he wasn't? The villain of the movie, Parallax, is a former Green Lantern who succumbed to temptation and turned into a giant cloud of evil yellow fart dust that is currently marauding through space, consuming entire planets in its wake. How did the Green Lantern Corps know that the same thing wouldn't happen to Hal Jordan? He quit immediately, which doesn't speak volumes for his character, and they let him take the cosmic power ring home with him anyway. That's like letting an Army recruit wash out of basic training with a helicopter and a briefcase full of nuclear launch codes.
Especially when you consider that one of the first things Hal did after he found the ring was use it to commit a triple homicide in a bar parking lot.
#2. After Earth -- Terrible Military Strategy Causes Thousands of Deaths
After Earth is set in a strange and distant future wherein the human race is forced to evacuate Earth and resettle on a planet called Nova Prime, which is eventually attacked by a race of aliens called the Skrel, who unleash a tide of monsters called the Ursa, who hunt down humans by smelling their fear (because this strange and distant future was apparently written by 9-year-olds).
In the first draft, Nova Prime's orbit makes all the women's boobs grow bigger, and people can fly by fart propulsion.
The aliens nearly wipe out humanity, but then General Cypher Raige (see "written by 9-year-olds," above) figures out a way to stop them -- he develops a technique called ghosting, wherein you eliminate your smellable fear by concentrating really, really hard, and the world's stupidest war is won.
But Wait a Minute ...
Scientists do believe that there's a fear pheromone in humans, and that it's secreted through sweat. With that in mind, take a look at this shot of Cypher Raige demonstrating ghosting:
And Will Smith failing to demonstrate acting.
Ignore Will Smith's absurd future haircut and look at the other soldiers. Every single one of them has exposed skin. Sweaty, smelly skin. Why the hell didn't they just wear odor-shielding coveralls?
The humans knew the Ursa (the people-eating monsters created by the aliens) hunted by smell. If they had covered up before going to war, the Ursa would've had no way of detecting them, and the campaign would have been over in days. Just send all of the soldiers out in wetsuits and paintball masks with satchels full of Axe body spray in case of emergency.
"We have Axe, Hawaiian flame shirts, and fedoras in the cargo hold. Use those and nothing will be interested in you. Ever."
Hell, you could've sent them out in fucking astronaut suits -- it wouldn't matter if the soldiers were clunky and slow, because we clearly see that the Ursa are completely helpless when they can't smell their targets. For some inexplicable reason, the aliens decide to engineer a bunch of blind, gunless monsters to destroy humanity, and the only edge they have is being able to smell fear (we will refer you again to "9-year-olds," above).
#1. The Cabin in the Woods -- The Facility Was Designed by Morons
The Cabin in the Woods is a generic horror movie where generic teenagers get killed off by generic monsters, until you reach the big plot twist, where it's revealed that their deaths are being orchestrated by generic villains in a generic secret underground facility to appease an even bigger generic monster (92 percent Certified Fresh on Rotten Tomatoes).
At the end of the movie, the surviving teenagers discover the secret underground facility and stumble upon a "System Purge" button in the lobby. They press it, releasing a swarm of werewolves, zombies, vampires, giant snakes, aliens, evil clowns, and basically every other monster you can think of from their respective holding cells in the facility. The monsters proceed to slaughter everyone inside Horror Base, which leads to the downfall of the human race.
But Wait a Minute ...
Why in the possible fuck is there a System Purge button, and what in the unutterable shit bassoon is it doing in the goddamned lobby?
Seriously, what possible reason could there be for installing a button that releases all the monsters at once? Did they plan on cleaning all their cages simultaneously? Do the demons and trees that rape people to death get lonely and need to be taken for walks? "Purge" implies they might need to quickly kill all the monsters. So why doesn't it activate a dozen flamethrowers in every cage, or bathe the entire joint in white-hot nuclear fire? Why does it empty them all out into the elevator bay like they just clocked out for lunch?
"Hey, you want to try that new Thai place?"
"Nah, I'm just going to grab some Carol from accounting real quick. I'm behind on my reports."
Even if the button killed the monsters instead of letting them all out, what architectural mastermind had it installed in the lobby? It isn't even locked. It's just sitting there in the guard house like a vending machine. We're surprised nobody ever pressed it by accident. We get that The Cabin in the Woods is making fun of horror movie tropes, but you can't make fun of dumb movies just by making your own movie infinitely stupider.
20th Century Fox, Ketchup Entertainment
Well, you can. But no one will respect your decision.
Related Reading: While we're talking about the dumb things smart characters get up to, has anyone else wondered how Ripley didn't realize she was being set up in Aliens? And if Hans Gruber was such an evil genius, why was his escape plan so woefully inadequate? On the opposite side of things, some of our dumbest-seeming celebrities are actually pretty darn bright.