Stupidity: Shriek is being taken to a new facility for superbeings, after a medical exam established that her sonic powers that she has been using since she was a kid are too strong to keep her at the correction house. So "naturally" for this transport she is not gagged, sedated, not even bound, and there's just one guy with her, not even wearing earplugs. It couldn't possibly be any more comically unsafe.
Stupidity: In the opening scene with the military convoy, the two soldiers in the lead vehicle never notice the car veer into their lane, even though their faces are bathed in bright light from the car's headlights. They simply ignore it and keep talking to one another.
Stupidity: While Tommy and Snake Eyes are trying to escape the docks in the truck, the Yakuza men surround the truck and stab over a dozen swords in all parts of the cab. Not one of the men ever thinks to use their sword to slash the tires, allowing the heroes to make an easy escape.
Stupidity: We know that the two 'special visitors' have been in our universe longer than a day. Despite being capable, smart, heroic figures, they did diddly-squat until the plot says so, since they haven't tracked down the very public (they recognize them) partners of Spiderman, they don't show up for the battle broadcast by JJJ on giant screens, but more importantly, they do not know who the "Avengers" are, showing they didn't look into Peter's history - the name would have popped up in relation to Stark, the blip and much more. Seems that they didn't even try to look for him.
Stupidity: When the rebel leader talks to Rick Flag guaranteeing her help, she mentions that she will help to find "this Gaius Grieves", speaking as if it was someone unknown to her. He's been around the island (going to bars and surely not being reclusive) for 30 years; it's pretty difficult to think it would not be one of the most known people on the island, given his role, unique appearance and the small community.
Stupidity: After the fight in the woods Bond walks back a long way, instead of driving the Toyota he had left in the forest in full working order.
Suggested correction: The Toyota has had a major chase over very rough terrain, it may no longer be operable.
It was operating fine when they stopped it in the middle of the forest.
Stupidity: A major effort is put in the future to develop the time portal, so they can recruit people from the past to help fight the war in the future. Yet they never seem to think to share anything with the past about the creatures and how to kill them or find out where they came from so the can help either prevent it from happening or help save the future before humanity is wiped out. Muri does recruit Dan for this purpose but it seems wasted to only use 1 person for this much more important goal.
Stupidity: Arishem's plan with the Deviants hardly makes any sense; he sent out biological beasts that he can't control, to kill off the natural predators of every planet. Forgetting the fact that obviously it didn't work (but the movie does not say that) because Earth has always had predators, that's a terrible plan to begin with, since any ecosystem needs predators or the other animals will grow uncontrollably.
Stupidity: Liv and her five friends went to Willy's to set it on fire, but Liv insisted they first go inside to get the Janitor out. All were aware of the situation and danger related to the mass murderer and the animatronics, but Bobby and Kathy went into another room to "hook up." Kathy recognized the room as the one where the killers committed suicide and later saw movement, but she continued having sex with Bobby until he was attacked and killed. (00:38:48 - 00:42:02)
Stupidity: To save the people from rubble falling on their heads Flash does not move them aside like he did with his future love, but does some strange manoeuvre which takes seconds - since we see people moving normally - and happens in midair (how does he get there?) and that does not take care of a huge chunk of concrete which he does nothing about, shouting just "No!", but Cyborg has all the time he needs to load his Mega Man arm and vaporize it in midair. (02:00:00)
Stupidity: Natasha is in Norway on the run - Norwegian radio mentions her by name, Yelena talks about her being on magazines, she's very recognisable, and yet she goes into a gas station with her sister without changing her appearance in any way, and loudly talks about being an Avenger and killing people, right in front of the gas station attendant who for all she knows would be on the phone to the authorities the second she's out the door.
Stupidity: When the crew think that Dorothy is a robot with a bomb planted in her and is capable of exploding, they begin to think that she is about to explode and attempt to dodge it. Rather than hide somewhere or take cover behind an area that may protect them, they just get down on the floor in front of them and cover their heads with their hands. If they thought the explosion was going to be as deadly as the one they saw on the news, this method of taking cover wouldn't be even remotely effective. (00:24:40)
Stupidity: Jon and his sister drove some distance to Bolton to find Janet, but Jon couldn't remember the name of the mini-mart where Janet works. Jon's sister said, "Maybe we should visit every market in Bolton", to which Jon replied she was a genius and "that's what we'll do." Instead of finding a telephone directory and making calls (first starting with Janet Brown!), they drove within a 10-mile radius, stopping at every store (not just mini-marts), but still could not locate Janet. (01:19:55)
Stupidity: The "body armor" General Ford and others wear is very bulky and appears to be quite heavy with all the attachments (e.g, flashlights on shoulders). With the protective suit protruding perhaps 8", it would most likely restrict/slow movement, contrary to what soldiers need. Moreover, the body may be protected, but there's no protection for the face/head, leaving them exposed/vulnerable. Today's products tend to be lighter, thinner, and stronger, so would be expected in the future.
Stupidity: Royce, blinded in Afghanistan, managed to avoid serious injury when the boat finally hit land. Having no real idea where he was, he foolishly decided to get off the boat and try venturing through unknown woods with the dog Chinook as a guide (!) This was simply absurd! Even though rescue missions were delayed that day by the storm, he should have had enough sense to know his chances of being found/rescued (and being safe in the meantime) would be the greatest by staying in the boat.
Stupidity: Right before the movie's final battle, Sub-Zero taunts Cole with his wife's necklace and makes him chase him into a portal. Sonya was right next to Cole, and she did not come along with him. They had previously planned to take on Sub-Zero together, but apparently she just left him to chase Sub-Zero alone to an unknown location.
Stupidity: It was announced on the radio "to stay inside your homes and to only leave in a state of absolute emergency." Then the house shook from an earthquake. May's mom (Gran) called and said, "I'm not sure you should come right now... the roads aren't safe" and "it's not safe to be driving around out there, especially with the kids." May, who told her mom she had the trip planned, finished packing and said, "Kids, we're leaving." Before driving very far, a "monster" was near her car. (00:01:33 - 00:03:47)
Stupidity: After Casi changed the tracker on the Navigator, he was sneaking between vehicles to get out of the tow pound. The Whale came and stood in front of the two vehicles Casi was between. Casi stood up and, instead of turning around and running away, rushed TOWARD The Whale in an attempt to get past him. The Whale, much bigger, threw Casi against a vehicle and then onto the garage floor. (01:03:34).