Stupidity: In an empty sewage tunnel, after Elsa saves Ethan, there just happens to be a defibrillator.
Stupidity: They make a big deal about how they've got the bare minimum of Pym particles left as Hank has been snapped away, but it never occurs to them to use their supply to jump back to a very safe time when Hank was around with a vast supply of them. They could bring them to their time and then have no end of attempts to get the stones without being on such a knife-edge.
Suggested correction: The problem is that they need to steal particles, and if they fail doing that, everything fails. They never thought of doing it that way because of that. Only when they failed in the past did they have to risk that, with again the possibility of failing. They couldn't take the risk to have their only chance of reversing what Thanos did fail because they want to be leisurely about it, ironically. They could do it in one go, that was the best bet and lowest in risk.
Stupidity: The bad guy who spies on Bosley is visible from the very beginning of the scene, when Elena is not even inside the cafe yet. The cafe does not have many customers and the bad guy is using a loud vintage typewriter with a big mirror mounted on top (!). The Angels are portrayed as being super smart and their setup is so careful, but somehow they managed to miss something amazingly obvious. Conversely, nobody would have ever paid attention to a laptop or any silent, modern-age device perhaps with a camera or something similar, so the bad guy chose the most blatantly conspicuous accoutrement for his spying job. Try showing up to a cafe punching the keys of a big clunky typewriter and literally everyone will be looking at you. (00:18:25)
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: 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: 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: 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: After so many failed attempts to open the large sea doors, why didn't the apes first breach the facility by climbing to the top of the cliff and simply enter through the existing rusted out exhaust vents where they escaped the rushing water?
Stupidity: After hearing the question on the radio station to win the Turbo Man doll and a failed attempt at the pay phone to answer it, Howard and Myron end up running two blocks to the station to answer the question. This is really stupid because the prize was going to the first caller with the answer and in the few min it would take to run to the station two blocks away and finally reach the DJ, many calls would have come in and most likely someone would have won already, way before the men arrived there.
Suggested correction: This isn't a stupidity or a mistake. What else did you expect them to do? Plus, the DJ even later asks if they expected him to actually have a Turbo Doll in the studio, and they both said "yes."
Not only that but, Myron had ripped the phone receiver out so Howard couldn't give the answer to the DJ. Howard and Myron running to the radio station was the only other option they had. At that point, it would only be a matter of if they got to the station in time before somebody could call in with the right answer.
It was a very easy question naming santa's reindeer. Although some people may not quite know them all... most people do. Now if it was a relatively hard question, our two guys probably had a better chance. But all good points made here in all these comments.
Stupidity: The prison where Magneto is held should have had a contingency plan just in case of escape, like machine guns outside the cell, or tranquilizer darts, or gas.
Suggested correction: What good would machine guns and tranquilizer darts be against a man who can literally control and manipulate metal? As for gas, the area outside the cell is so large that any gas would quickly dissipate before it could be effective.
Stupidity: The lead Predator got infected with a Xeno chestburster, and at the end of the movie, his body is shown on the ship with it bursting out. This is what sets up the sequel. However, the Predators have been coming to Earth for centuries, and they have also been hunting the Xenos for centuries and know their behaviours and breeding habits. They have X-ray capability and even use it in this film to show a human is infected. Yet they never scan his body before bringing it on their ship. And yes, I know there's the whole thing about them coming out victorious, it seems, and the other Predators likely would assume they were untouched. That doesn't make it not stupid that they still didn't scan his body to be sure, as they know what these things are capable of. And of course, in the sequel, that stupidity bites them in the ass.
Stupidity: All the mutants on the island need Magneto at the last second to take control of the missiles so they don't land and explode. Why didn't Azazel just teleport everyone off the island?
Suggested correction: Given the situation, there was probably very little time to come up with a plan like this. The missiles were flying towards them extremely quickly, and there would have been nowhere near enough time to band every single person together for Azazel to teleport them all out of there in just a few seconds.
There was plenty of time. He could have taken 5 at a time down the Beach 3 miles and came back for the rest. Earlier in the movie he was moving so fast men were falling from the sky at nearly the same time and he was teleporting them one at a time.
Stupidity: The coast cities should have an alarm that detects the Kaijuns. If they do, they should work when the beasts are few miles away from the shore in order to give the people time to get to the shelters. Instead of that there are still people on the streets in Japan and on the docks in Los Angeles (in the flashback) while the kaijuns arrive from the sea.
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: 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: Any fighter pilot should know better than to attempt a low flying attack so close to a hill, let alone into a tunnel. How could he not know that it was there? How'd this moron even get his pilot's license?
Stupidity: In the shuttle, when the fountain pen is floating in zero gravity, the flight attendant grabs the pen and puts it in the pocket of the sleeping passenger. She doesn't put the cap on. She puts it in his pocket nib down. Even though there is no gravity, the capillary action of the nib touching the shirt will pull the ink into his clothing and stain his shirt and jacket. (00:19:40)
Suggested correction: Who said it's a fountain pen? 2001 is set in an imaginary future with imaginary technology, one of which might be leak-proof pens.
Stupidity: The system prompts a message for the bank clerk saying that the subject trying to withdraw cash is armed and dangerous and to alert the local authorities. It also alerts the 'subjects' themselves though, since it beeps a persistent alarm tone! Seems a rather poorly designed system that would endanger the staff. (00:50:10)
Stupidity: When the mayor's daughter asked for water and it was found to be undrinkable, surely the other residents in the town would have found that out for themselves well before the 6pm meeting at the gymnasium and made the necessary arrangements to leave.