Stupidity: How on earth did those helicopter gunship pilots survive a tour of duty in Vietnam? They are too stupid to tie their own shoelaces. They have encountered a thirty metre tall ape which is intent on killing the lot of them by grabbing their helicopters and smashing them to the ground, so they have two clear choices - a) use their heavy, mounted machine guns or the grenade launchers fitted to the assault rifles carried by the troops on board to shoot the thirty metre tall ape to ribbons from a couple of hundred metres away, well out of its reach or b) fly straight up to it and present an easy target, allowing it to kill the lot of them by grabbing their helicopters and smashing them to the ground. Every single one of them ticks box b). Idiots.
Stupidity: When Flint Marko is escaping from the police, he climbs a chain link fence into a test facility. Besides having a chain link fence, there should have also been security cameras around the area as well as guards to prevent anyone from trying to get in. Plus, when the scientist notice on their computers that there's an increase in mass, one of them simply assumes it's a bird and then they continue their experiment. Instead of making assumptions, they should have halted their experiment and have someone take a look to see what caused the increase of mass.
Stupidity: The Death Star comes equipped with a powerful tractor beam capable of capturing a ship the size and agility of the Millennium Falcon. Why don't they use it against the rebel fighters attacking them at the end of the film? Okay, Obi-Wan Kenobi turned it off earlier but I find it hard to believe that someone who has never before visited the largest, most complex space station in the Universe and who was previously unaware of its very existence can disable a fundamental security system but the people who designed, built and run the whole thing can't work out how to switch it back on. They should have no problems with this, considering the fact that Obi-Wan didn't damage it.
Stupidity: Ultimately, it did wind up working. However when they shoved the antidote vial into Claire's purse and got George's attention so he would eat her, this was an insanely stupid move. 1. They had no way of knowing if he would actually choose to eat her if he even grabbed her in the first place. Before then he'd been crushing people and even threw the helicopter pilot miles off the building while they watched that happen. 2. The antidote was in a plastic polymer like tube designed to hold the substance inside it while submerged in liquid nitrogen containment. Even if the gorilla's stomach acid could eat through the vial to get to the antidote, it would take hours for it to get through a material like that. 3. Gorillas are herbivores, so it wouldn't have tried to eat a person in the first place. (01:17:55)
Stupidity: During the execution scene when Carina is on the gallows with a noose around her neck, the plot depends on Henry being able to get to her before she is dropped through the trap door. However, there is so much slack in the hanging rope that during the chaotic fighting, when Carina is left unattended, she could easily have stepped off the small trap door and onto the solid flooring that is inches away. Instead, she continues standing there, waiting to be rescued.
Stupidity: When they first arrive on Planet Zero, Victor runs his hand through the mysterious green liquid on the planet surface. No scientist in their right mind would ever willingly attempt to touch a body part to an unknown substance. He had no way of knowing if the substance was toxic, corrosive, incendiary, hypothermic, etc. Moreover, none of his companions try to warn him against doing so.
Stupidity: Steve Trevor is the leader of a group of Allied spies, and they are traveling with a gorgeous woman who has shown to have amazing fighting skills and super powers, but they allow a group photo to be taken with Diana in her armor. They don't know who is taking the photograph nor what they intend to do with it. Their photograph circulating through a news publication or passed on to an enemy's intelligence service could compromise himself and the rest of his team, and they would absolutely know that. Undercover agents do not pose for photographs.
Stupidity: The Engineer and Fireman of the Polar Express train crew should have known better than to not bend back the the metallic prongs of the cotter pin, which would have prevented the cotter pin from coming loose and causing all those problems in the driving of the train.
Stupidity: After the team break Solomon Lane out of custody, they drive around Paris with him in the front seat of their car. Lane is still wearing a straightjacket, bound in chains and has a black bag over his head. The IMF team drive him through crowded streets, past multiple police cars in full view of everyone just minutes after an armoured convoy transporting a dangerous prisoner (who everyone is now looking for) has been attacked. None of the Parisian police seem to notice this and none of the IMF team questions the tactical soundness of this approach.
Stupidity: When Anna, Velkan and the villagers successfully trap the gray werewolf in a huge cage, the villagers start firing on it. Anna says they need to use Velkan's gun because it has silver bullets in it. Since everybody knew they were fighting a werewolf, all of them should have had silver bullets for their guns.
Stupidity: Beck wants to kill Peter's friends because they know his secret. Instead of using Edith to attack them directly with a drone strike, or using his illusion technology to lead them into the path of a train like he did with Spider-Man, he instead has a henchman drive them onto a bridge and leave them in the path of his next Elemental attack. Because absolutely nothing is forcing them to stay on the bridge, they all casually walk off the bus and out of immediate danger. It is unfathomable that a man as intelligent and resourceful as Beck would take such an idiotic approach, especially considering all he had at his disposal and how desperate he was.
Stupidity: The commando mission to save Chewbacca starts gunning down a few Stormtroopers in the hangar. The heroes then go on leaving the troopers lying down on the floor in front of the ship, in plain view. They don't hide them nor ask the droids (who have enough strength and tools to pull them in) to, in fact they tell them to stay put. No wonder they are found out later (after a ridiculously long amount of time).
Stupidity: Bori Khan is a skilled archer and a man without honor. He's not fighting fair, and cares just to win, as established and explicitly said. He shoots an arrow at Mulan, the Witch gets in the arrow's way sacrificing herself to protect her...and for no reason whatsoever he does not shoot anymore, giving the chance for the two girls to share their very special dramatic moment together, but creating a colossal plot contrivance. It is a know movie cliche for the fight to 'pause' around the main character, but here we have a sniper who desists completely (he won't shoot anymore) for absolutely no reason. (01:29:00)
Stupidity: When the Keymaker is closing the door to the room that leads to the Source, he stands in the doorway resulting in the multiple Agent Smiths gunning him down. He could have easily closed the door without standing in the doorway and consequently would have lived.
Stupidity: Reliant's prefix code is 16309. This code prevents an enemy ship from ordering a friendly ship to lower its shields or something similar. Five digits with no symbols. We know that symbols and letters aren't used since Spock uses a 10 digit set of switches to input the code. This is ludicrous. In 2016, a high powered server could crack a 6 digit password in approximately 0.0224 seconds (at 100 billion guesses/second). Any starship computer would have to far more processing power then a 2016 server. The prefix code protecting a starship from cyber attack would have to be insanely complex in order to be useful.
Stupidity: When Colonel Hardy is about to crash the ship he's flying into a building, Faora sees it and she has more then enough time to leave the ship and get to safety, but she does nothing and just stands there.