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: 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: 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: Kord Headquarters goes into "lockdown" after the Scarab is stolen, which in this building, apparently means allowing all the guests to leave.
Stupidity: When the shuttle arrives with the assassins, O'Niel is shown loading a shotgun, then he carries that gun when he runs to his office. When he leaves his office again to confront the assassins he grabs another shotgun from the locker. Why didn't he just grab the gun he walked in with?
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: 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: The ship crashes at the end after the reverse polarity from the shuttle causes an explosion. This could have been avoided by a) doing a spacewalk with a multimeter and checking the connections or b) not connecting the shuttle to the mothership. And it is hard to believe that there were no circuit breakers or fuses that would blow and protect the ship.
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: 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: They could've used the flying chopper thingy to kill off the armed guys when rescuing the kids off the train carriages. That way they could have made sure they got every single kid rather than rushing to just take the one containing Minho (which they messed up).
Stupidity: There was no particular reason for Nuclear Man to take Lacy into the space in the first place.
Stupidity: One of the actors microwaves bullets and then loads one into a weapon. Only problem: she/he (I did not pay attention) puts the round the wrong way around into the magazine. Then the person loads the magazine with the one round visible in the wrong direction into the weapon. I would say that this should not be working. (04:25:00)
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: 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: Given his speed, surely Superman could've stopped both missiles with no problem.
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?