Stupidity: When Rumsfeld is on his roof as a look out, he has a clear view of the neighborhood, even at night. However when the Klopeks first come home that night (and realise that someone is in their house), Rumsfeld fails to notice their car until they return again with the police. (01:20:00)
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: 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: 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: When Wolverine is trying to prevent Deadpool from sacrificing himself by destroying the time ripper device, he repeatedly pounds his fists against the glass panel on the door to break it, to no avail, which leads him to then go to another heavy door, which he is only able to break down in the nick of time. Wolverine's adamantium claws would have easily destroyed the glass on the other door, without him having to repeatedly and futilely punch it.
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: 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: 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: 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: During the film's climax where Santa and his reindeer successfully ascend over Manhattan, the sleigh's rocket engine falls off and apparently is no longer needed. Isn't it rather risky (and foolish) to assume (or hope) that the other cities on his flight path have enough "belief power" to sustain him aloft for the rest of his journey?
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: The whole movie happens for unbelievably convoluted causes. Despite their friendship and the simple fact that he still runs an occult shop (showing that he's not exactly insensitive to the past), Ray in years never spent a single moment to check on Egon's whereabouts, which he knew, or listen to him. The city has Shandor's name all over and any cursory investigation would have unveiled the connection. Assuming of course that Egon turned into such a lunatic he couldn't do the namedropping himself.
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: 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.