Stupidity: Why in the world would Tony of all people leave his old suits in storage WITH the miniature arc reactor still inside the suit? This would have served no purpose to Tony since he already has his own arc reactor in his body.
Suggested correction: This is a question which is easily answered; Tony had the idea of putting someone else in the suit (like Rhodey) himself. Plus, it's logical to think he has built multiple miniature arc reactors, after the events of the previous movie. Plus, he can probably just as easily switch from arc reactor to arc reactor in his chest when changing suits, so he has a fully powered arc reactor ready in each suit.
Stupidity: When the Woolworth Building collapses and the gang runs into the convenience store to take cover from the dust, Marlena was last seen behind Hud and with Lily, Rob, and Jason. Switch to everyone running from the dust cloud, Marlena didn't make it into the store. Upon exiting the store, the gang finds Marlena stumbling from nearer to the Woolworth Building meaning she would have had to run toward the dust cloud, through the screaming crowd which was running away from the building.
Stupidity: Central to the plot is the layout of the Mars lander. There are no seats or any restraints of any kind in the lander, which is ridiculous. Spacecraft require safety belts in order to prevent astronauts from being injured during rapid course corrections. Additionally, ordinary rolling desk chairs would not be sent into space, and would certainly not be placed on a landing vehicle.
Stupidity: The Analyst wishes to hide the true identity of Neo from anyone inside the Matrix, so his appearance is altered. But in this new Matrix Neo is also named Thomas Anderson, his name from before he was pulled from the Matrix originally, and he is a famous game developer that created a video game series called "The Matrix." It is frankly astounding that anyone actively looking for Thomas Anderson would not immediately see through this ruse and instead would be fooled by balding hair.
Stupidity: After Corinne Dufour realises that Drax intends to have his dogs kill her, she inexplicably attempts to flee on foot, running right past the golf cart she arrived in. The golf cart would have given her more speed, range and protection, and would have been a much better choice for fleeing.
Suggested correction: Golf carts are very slow, and are open on all sides: the dobermans would've easily dragged her off it. The biggest stupidity here is that she should've fled to a crowded area with other people instead of the woods. Then again, she was panicking, and not thinking straight.
Stupidity: Right before the movie's final battle, Sub-Zero taunts Cole with his wife's necklace and makes him chase him into a portal. Sonya was right next to Cole, and she did not come along with him. They had previously planned to take on Sub-Zero together, but apparently she just left him to chase Sub-Zero alone to an unknown location.
Stupidity: Wouldn't it be common sense to have more than one remote to control Robo-Cain? That way, if he destroyed one (and he did), they could still use one to shut him down from a distance (as long as they can dodge the gunfire).
Stupidity: In the Anubis universe, immediately around the time Lawless says, "Something's wrong," if you look closely, by Lawless, you see the two police officers behind Jet Li walk away to leave Lawless completely unprotected from behind. I understand it's a parallel universe, but that's some really bad police work to leave a prisoner wide open like they do.
Stupidity: After Noah shoots the Humanity Bureau vehicle following them, they continue down the road and stop at an old abandoned building that still has some sort of running water in it. Lucas has to go to the bathroom so his mother just lets him wander off by himself to find a restroom. She doesn't go with him or make sure he's fine. And this is in an area where they know that most of the residents are hostile, and they are being chased by the Humanity Bureau. Noah even calls her out on the stupidity of her letting Lucas go by himself and she claims it's because he needed to do number 2 and that Lucas insisted and he's not a little kid any more... Which doesn't excuse the stupidity of the action of letting an 11 year old go by himself to find a bathroom in such a situation. And ultimately it does lead to him getting found by Agent Westinghouse. (01:08:30)
Stupidity: The bullies run their car straight into someone they think is a total wimp (Goku) at full speed, crushing his bike. It's not a dystopian future, had Goku been a normal person they would have straight up murdered / crippled him for no reason at all. They are portrayed as a group of jocks, not homicidal maniacs committing roadkill on their way to school. It has also to be noted that Goku was parking his bike in a car parking spot, where nobody would just put a bicycle - but it is needed by the plot so that the bullies can casually crush his property. (00:06:35)
Stupidity: Harrigan leaves his partner's badge on his grave where anyone could take it and use it to impersonate a cop or abuse it in other ways. (00:50:15)
Stupidity: I won't begrudge Father Lamont for instinctively grabbing for the first thing in reach to try to smother the box on fire (even if it is a pair of wooden crutches), but you'd think at some point he'd notice that he's just spreading the fire and making it much, much worse, but he keeps at it until the doctor finally returns with the extinguisher.
Stupidity: While the train cars are still derailing in front of them, the kids stay on the porch and actually start running toward the oncoming train. Next, they turn around and run in the opposite direction, while staying the same distance away from the train tracks and flying cars/debris. When they run out of the porch to stay parallel to the train, they again turn around and run to the middle of the porch and look like they are jumping around together, clueless where to run to next - running in either direction parallel to the train was not getting them away from derailment. Then they disperse in various directions but stay off the porch. (00:17:24)