Stupidity: After Julie tells Louis that Peyton is still alive, she notices the Bellisarius memorandum on his desk and realises he was involved with the destruction of the lab. If Louis had kept the memorandum in his desk instead of in plain sight, he would never have been found out.
Stupidity: McCord gives Lincoln and Jordan a credit card and tells them that he will have to report it missing within twenty four hours, and he warns them that they can be traced by using it. He then gives them a huge wedge of cash. Since we have a society that still uses cash, and he has acknowledged how dangerous the use of a "stolen" credit card is - he even tells them how using it will lead the authorities straight to them - why not just tell them to use the cash? (If there wasn't enough, they get a cash advance on the credit card up to its limit - easy). He repeatedly acknowledges the risks of using the card, and it is absurd to speculate that he didn't have time to find and use an ATM to max out the card. He doesn't have to find an ATM - Lincoln and Jordan do, and as he acknowledges they have twenty four hours to do so. Once the cash is withdrawn, the card is thrown away, and nobody is at risk.
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: John, the guy who taught Chucky the voodoo spell is confronted by Chucky later on in doll form. When things don't go how Chucky wants, he finds a Voodoo doll of the same guy just lying around and uses it to torture him. So this idiot who knows Voodoo and what it does, happens to have a Voodoo doll of himself just lying around? Yeah sure, that can't go wrong at all.
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: When Clark is fighting Evil Superman in the junkyard, Evil Superman looks up and sees a huge magnet directly over Clark and Clark notices it too. Evil Superman uses his heat vision on the chain causing it to heat up then break dropping the magnet on Clark. Clark had plenty of time to get out of the way before the chain snapped but he just stands there when it finally falls on him.
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: Someone with the Punisher's tactical knowledge would not hang upside down from a chandelier to shoot in a 360-degree pattern at his targets. This would make him a sitting duck and it's only because of the movie tropes of Stormtrooper aim and cannon fodder henchmen that he isn't killed because of 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: 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: The entire plot hinges on a military facility that knows it's housing what's essentially a sentient world-ending disease, and that has the foresight and resources to build a containment room with a cyanide failsafe, having nothing else to stop said virus from breaking out beside the glass her room is mostly composed of. Once she's out of that room, she just has to run through an unlocked, labeled exit door and hop a fence. Where are the emergency bulkheads? Where are the armed guards watching the hallways? Why isn't that fence electrified? Why isn't she wearing a metal shock-collar at all times? Even if that glass was bulletproof and/or they seriously underestimated her strength, they'd still have to open the door to her room at various times to give and take her food, books, laundry and waste.