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 Roy finally gets to the Lima project in the shuttle but can't dock, he just lets it drift away - he could have at least tethered it to the station to use it for his return trip.
Suggested correction: There is no way to know the reason why he did this. It's plausible, or even likely, that the pod was out of fuel. Not only that, but it could not dock with Lima because it was damaged, so it's likely that the damage played into the decision as well.
Stupidity: The characters wish for the resurrection of just one person (Roshi) rather than everyone killed by Piccolo as would be natural (and as they do in anime and manga). Okay, Piccolo in this movie sucks at killing and resurrecting Muten covers 50% of the total body count, but they can't know that.
Stupidity: They go to great pains to lock the nodes behind an explosive lock, then have a debate about whether Riddick's somehow broken in. They eventually unlock them, check they're still in place...then just leave the locker completely unattended and unlocked while they discuss plans, and Riddick of course steals the nodes.
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: 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: During the auction, we see the dinosaurs are brought into the room and placed in the middle. Thus blocking half of the bidders from the auctioneer's view. (01:16:40)
Stupidity: Colonel Sharpe retrieves a gun from a locker on the Freedom shuttle. However, said gun is shown lying flat on the floor of the compartment, with no visible means of securing it, which would hardly be standard NASA procedure given the shuttle undergoes a high-G launch, orbital manoeuvres and various other high acceleration events. A loaded gun bouncing around is the last thing anyone would allow on a shuttle. (01:42:39)
Stupidity: Having become the leader of the universe's most stealthy fighters, he promptly leads them in a Custer-esque noisy frontal charge into a force with known weapon and technology superiority. Zero effort to utilize ANY of their amazing stealth, just have them slaughtered so badly that the planet has to sacrifice animals to save them from extinction.
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: There was no particular reason for Nuclear Man to take Lacy into the space in the first place.
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: 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 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: 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: Why didn't they take out the crushed hand guy while the creature was fumbling inside the glove? Plenty of time to do it. Why did no one use a knife to stab the creature?
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: In the freezer you can see the electrical wire power cord that is powering the motor on the wall. She could have easily pulled/ ripped the wire to stop the freezer from working. And when the chemical bomb is in the mixer they could have unplugged the machine which would stop it from going off.