Stupidity: There is no reason why any person as intelligent as Janet would keep the knowledge of Kang secret from her family. The extended Pym family are the only people in possession of the one thing Kang needs to escape. The brief explanation she gives is that she wanted to protect her family, but this makes absolutely no sense whatsoever, and she makes no attempt to explain how this secret keeps anyone safe.
Suggested correction: She is obviously scared out of her mind concerning Kang. She, through her fear, had hoped that him being trapped in the Quantum Realm would stay permanent as long as nobody knew about it in the normal universe. In that way, she tried to protect not only her family but the entire universe.
Not only does she not say that she is "scared out of her mind", she also doesn't act like it either. There is no indication that she is so frightened by Kang that she has lost her senses - quite the opposite, actually. She appears to function rationally and intelligently in every other area concerning Kang, except of course for simply telling anyone how dangerous the Quantum Realm is because the movie wouldn't have a plot otherwise. It's pretty egregious and wildly ridiculous.
Of course, she doesn't say that or act like that. But what she saw of him, when she touched his ship, scared her enough to go to all that trouble to keep him in the quantum realm at all costs. She thought it would be safe to leave, that he was trapped forever. Her judgment was wrong, probably caused by her fear. She is only human.
"Fear" is not enough to get past this level of stupidity. My point is that she doesn't act so frightened; she isn't irrational in any other way. It's just a flat-out, stupidly written element of the film that is impossible to believe. There is no way on God's green earth she should keep this secret, even after her family has made it to the quantum realm. I get that the movie is trying to say she is frightened, but this goes well beyond making any kind of sense at all; it's ridiculous.
Part of the stupidity also involves Janet's action in the mid-credit scenes of "Ant-Man and the Wasp," where she actively helped send Scott into the Quantum Realm to get quantum energy. If she was so afraid of a signal being sent to the QR, she wouldn't have let Scott go without explaining the dangers of going. This film seems to ignore that and instead seems to focus on Janet simply not wanting to discuss her involvement with Kang and her guilt, thinking no one would go back to the QR.
Stupidity: Al is supposed to be a super serious toy collector and seller, but he handles valuable toys without cleaning the cheese puff dust off his hands.
Suggested correction: It's also established someone is coming over to clean said toys, dust included, and when he does the handling, he's just been sharply awakened by the TV therefore is off-kilter.
Stupidity: The whole premise of the plan is summarized by Helen in the sentence "They don't know Andi is dead, so why would they suspect anything?" However, "they" conspired in unison to financially ruin her sister, and "they" did not answer the mail, none of them. The possibility that "they" all could have gathered and silenced her by force is one of the most likely ones, and yet neither she or Blanc consider even for a moment that the literal crime conspirators could have conspired.
Stupidity: The train which takes M from New York to London arrives looking like an old tube train, then when she's on board converts into a high-tech train, to her amazement, and whisks her across the Atlantic. Except...why did it ever look like a regular train? She's in a MIB station, it's only used by agents and aliens, it goes to another MIB station, and when it arrives and people get off / on before it continues its journey, it stays in its high-tech form anyway.
Stupidity: When Susan is on the balcony with the man trying to kill her, she pulls out her gun with one hand (she's leaning on the other hand). He says she won't shoot because the gun's magazine has fallen out and Susan gives up and drops the gun. However, if there is a round in the chamber, the gun can still fire without a magazine in it. As trained as Susan is suppose to be, she should know this. But, if there's not a round in the chamber, then, as a trained agent, Susan is ineptly prepared to use her gun. If there's not a round in the chamber, pulling the trigger will not fire the gun nor will pulling the trigger chamber a round. But she would need both hands to chamber a round and she only has one hand free. There is the occasional gun that has a magazine disconnect safety, but it would be stupid for field agents to have one since they may be in a situation where they need to fire without a magazine in (such as this situation).
Stupidity: The idea that to stop someone from reading an e-mail if you can't sneakily delete the mail at least you can steal their computer altogether is flawed (for one, they can access their account from a different machine - and turns out Jules' mom has two laptops at home), but despite that, and the fact that everyone in Ben's team is supposedly tech-savvy, nobody takes exception to that. (01:10:20)
Stupidity: Everyone laughs off any reference to the supernatural and the Ghostbusters being any more than hacks. However, they drilled a hole that unveiled a literal river of pink glowing goo that is even brought up at the trial; nobody investigates its origin or acknowledges the fact, which is exceedingly absurd since it involves heavy pollution of the underground system of a metropolis.
Stupidity: When Matt goes to the bank to close out the account he learns that it's been closed already by Kelly. Matt says he will be expelled, but he never did anything wrong except lie about who Kelly was. The bank teller is the one who was flirting with him and gave him the money with out any proper ID or papers, so if anything the bank would reimburse Matt/the school.
Stupidity: When the bellhop is in the elevator and it looks like this is where he loses his arm, he spends a few minutes trying to stop the elevator. All he had to do was let go of the luggage and he would have been able to easily bring his arm in.
Stupidity: Knowing that the Klowns' noses are their weak point from previously shooting them, Dave doesn't bother to shoot the giant Klown at the end with his pistol, instead waiting to be almost crushed and finally popping it with his badge pin.
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: Jack has been using Mosley's badge and identification to pass himself off as FBI. He never once uses this trick to commandeer a civilian car for him and John when it would have been the safest way to get to L.A. much sooner.
Suggested correction: Probably because If he did that, the car's owner like Red the bar owner would eventually call the FBI office to get their car back and then the feds would know the make, model, license plate, and the last location of Walsh and the Duke. The police would have caught them in minutes. Walsh had to keep a low profile.
Stupidity: When Dr. James Harvey dies, there should be a body left behind in the manhole. After Kat and Casper revive him in his ghost form in the Lazarus, he comes out with a body (and clothing). One can assume that there are now two bodies of him, one alive and one dead. That would make an interesting article in the newspapers of the next day.
Stupidity: The fact that Nolte even gives in to Eddie's demands is ridiculous. Why not just interview him in prison, ask him where to look for certain people, etc, and if it's a dead end, report back and ask for further tips, etc. They had zero problem going to prison and asking Kirkland Smith in the sequel.
Suggested correction: He refused to tell him anything unless Kates takes him out of prison.
Stupidity: It is pretty silly for Martin Lawrence to go undercover as a police officer when it would make way more sense to go undercover as a janitor/custodian since his main goal is to gain access to the vent. A janitor would be a way less conspicuous disguise than a detective and slipping through a backdoor entrance disguised as a janitor should not be all that complicated for someone as savvy as Myles. But of course this would make a far less entertaining film.
Stupidity: When Gail shoos Luke off the first day, in fact nobody accompanies him to the door and they discuss the evil plans when he's not even gone nor they even check he is, they just go ahead with the script. It's a rather odd dynamic, very rushed and movie logic-like. Or theater logic, since it's a typical 'exit stage left' moment.
Stupidity: During the ghost chase montage, why would Velma look for her glasses with her hand in midair instead than on the ground? No matter how nearsighted you are, the closest wall, or obstacle, is meters away and you'll never find your glasses anywhere but the floor. She does it for a longer time than anyone would. (00:52:40)