Stupidity: A-X-L was supposed to be a highly sophisticated and intelligent robotic dog yet it let Sam torch it (without even trying to back away from the flames) and let drones attack it on two separate occasions, each resulting in A-X-L being incapacitated. A-X-L also fell and rolled down a hill, smashing itself. An advanced machine should be able to stay upright! It also seems odd and out-dated that a robot would run on gasoline.
Stupidity: When the plane begins crashing, Kayla tells Claire that she is sitting in an ejector seat. She also mentions that there are no parachutes because she "wasn't expecting company." If Kayla was expecting to be alone in a crash scenario, what use would installing the ejector seat in one of the passenger seats have been? She would have to move from the pilot seat to the passenger seat while the plan is crashing. Surely it would've been easier to install the ejector seat in the pilot seat.
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: Whilst the chaplain and one of the marines are talking in the kitchen, a demon sneaks up behind the marine, stabs him through the back and kills him. The demon then walks up to the chaplain and strikes him across the face and leaves the room to search for the rest of the crew. Despite the intent being to kill the crew, the demon does not make sure that the chaplain is dead before leaving. It just assumes the strike across the face will be enough to kill him.
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: The creatures in the maze make it inside the living area after the gates open and some of the heroes hide in the corn field. While that alone isn't the smartest thing... one of them keeps a lit torch in their hand burning brightly at night while trying to hide from these creatures.
Stupidity: Dr. Cocteau's choice to unleash an enhanced Simon Phoenix without any way to restrain him is incredibly reckless and stupid. Even if Simon were to kill Eager Friendly, in the best case situation, you'd still have a madman with total computer access, martial arts knowledge, etc., that you would have no way to rein in. Sure, he can't kill Dr. Cocteau, but what would stop him from say, holding the city hostage or something? Why not add in a kill code or something to keep him in check?
Suggested correction: Who says he didn't? Cocteau has put in mental conditioning compelling Phoenix to kill Edgar Friendly and make him unable to kill Cocteau. Who says he hasn't put in something that makes him kill himself after the deed is done? Or perhaps paralyze him so he can be put back on ice. It's just that Cocteau didn't count on the fact that his henchmen could kill him. He doesn't care about how dangerous he is, not until he has done the deed.
Dr. Cocteau is a narcissistic egomaniac type that would see himself as a king or a god, even. And Simon is making him very angry. He even tells Simon, "you're beginning to be more trouble than you're worth..." Someone with an ego like Cocteau wouldn't stand for Simon's antics for very long. And would happily enjoy putting Simon back in his place by shocking, paralyzing, etc.
But he first needs him to kill Friendly. Until he does that, he'll let him play. He still sees no danger to himself.
Stupidity: All of Robbie's circuits have been burned out as a result of Morbius ordering him to kill the Id monster. The crew only has 24 hours to get the ship loaded and a safe distance away from Altair IV before it explodes. But one or more of the crew spends the little time they have fixing Robbie, a very sophisticated piece of technology, who is working the controls at the end of the movie.
Stupidity: The "body armor" General Ford and others wear is very bulky and appears to be quite heavy with all the attachments (e.g, flashlights on shoulders). With the protective suit protruding perhaps 8", it would most likely restrict/slow movement, contrary to what soldiers need. Moreover, the body may be protected, but there's no protection for the face/head, leaving them exposed/vulnerable. Today's products tend to be lighter, thinner, and stronger, so would be expected in the future.
Stupidity: Jake throws Derek overboard to distract the piranha, then swims under the boat and doesn't get attacked, so why couldn't he just go downstairs the normal way and have Kelly swim over to him? They could have escaped a lot more easily.
Stupidity: This highly trained squad of soldiers is sent into a known contaminated zone with all manner of chaos going on within in...and the second one of them sees a woman outside his APC he ignores a direct order and pops outside to see how she's doing.
Stupidity: The X-Men depart for space on a vehicle not designed for space, for an incredibly dangerous mission they know nothing about (and none of them can survive in space at that point), but don't suit up for it in the slightest - and with Nightcrawler and Quicksilver's powers as they are shown later, it'd be easy to do it without wasting time. Not only that, but Xavier communicates with NASA to get the briefing only after the X-Men have already departed.
Stupidity: The Foot Clan found the Turtles' hideout in the sewers thanks to one of their members who followed Raphael when he was carrying an unconscious April O'Neil after he saved her from the evil ninjas. When Shredder later is trying to find the Turtles, it takes Danny's betrayal to point him towards April. How stupid does he have to be? It's a connection anyone would have made, it was the second time she was saved, even.
Stupidity: Dr Faxx knew full well that Nuke was Cain's greatest weakness, but it never occurred to her to mention to the old man that holding a canister of it in front of him during the exhibition could have serious consequences. Not very bright.