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 Once the cash is withdrawn, the card is thrown away, and nobody is at risk.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: 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: If the song "Beyond the Sea" can be continuously broadcast on the radio station as a "cryptic clue" that there are survivors living on the island and the aliens are unable to swim, then someone could verbally announce that information over the radio and otherwise communicate with other humans. The same thing about how Regan's hearing aid incapacitates the creatures. Rather than explaining how to weaponized the high-frequency sound, only the hearing aid's noise is broadcast and anyone hearing it has to figure it out.
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: At the end of the film, the Rehabs recruit gang members and give them all kinds of guns from shotguns, handguns, and automatic weapons. The cops and deputized citizens have similar weapons. But for all the shooting, not one gang member is hit and only one citizen appears to be shot (aside from the three blown off the building roof by the Rehab tank.) I don't think anyone has any bullets.
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.
Stupidity: At the prison yard, when Weiss overhears that the sonic deadline was being deactivated to let new prisoners in, he manages to look at the deactivation code. The guard sees Weiss looking at the code and switches the computer off. If he had changed the code and informed another guard, none of the prisoners would have escaped.
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: 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: 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.
Stupidity: Monk steals the magazine from Coffey's SIG. However, I don't see how he could have jacked the slide to eject the round in the chamber without being noticed. Someone as paranoid as Coffey was at this point would not have left the chamber empty. (Just FYI, when you load an automatic pistol with a magazine, you have to pull back and release the slide to load the first bullet in the barrel.)
Stupidity: When Greg volunteers to watch the security monitors to see if the creatures can affect people through a screen, no-one even thinks to remain in the room with him to cover up the monitors with a blanket, towel or something similar in case the creatures' power can be transmitted that way. They tied him to a chair to reduce his chances of committing suicide, so they certainly thought there was a possibility that he would be in danger.