Stupidity: After Julie tells Louis that Peyton is still alive, she notices the Bellisarius memorandum on his desk and realises he was involved with the destruction of the lab. If Louis had kept the memorandum in his desk instead of in plain sight, he would never have been found out.
Stupidity: McCord gives Lincoln and Jordan a credit card and tells them that he will have to report it missing within twenty four hours, and he warns them that they can be traced by using it. He then gives them a huge wedge of cash. Since we have a society that still uses cash, and he has acknowledged how dangerous the use of a "stolen" credit card is - he even tells them how using it will lead the authorities straight to them - why not just tell them to use the cash? (If there wasn't enough, they get a cash advance on the credit card up to its limit - easy). He repeatedly acknowledges the risks of using the card, and it is absurd to speculate that he didn't have time to find and use an ATM to max out the card. He doesn't have to find an ATM - Lincoln and Jordan do, and as he acknowledges they have twenty four hours to do so. Once the cash is withdrawn, the card is thrown away, and nobody is at risk.
Stupidity: A character on foot when chased by a boulder, vehicle, etc., will often run in the same direction as the object, rather than off to the side where the object can't reach.
Stupidity: How on earth did those helicopter gunship pilots survive a tour of duty in Vietnam? They are too stupid to tie their own shoelaces. They have encountered a thirty metre tall ape which is intent on killing the lot of them by grabbing their helicopters and smashing them to the ground, so they have two clear choices - a) use their heavy, mounted machine guns or the grenade launchers fitted to the assault rifles carried by the troops on board to shoot the thirty metre tall ape to ribbons from a couple of hundred metres away, well out of its reach or b) fly straight up to it and present an easy target, allowing it to kill the lot of them by grabbing their helicopters and smashing them to the ground. Every single one of them ticks box b). Idiots.
Stupidity: When Flint Marko is escaping from the police, he climbs a chain link fence into a test facility. Besides having a chain link fence, there should have also been security cameras around the area as well as guards to prevent anyone from trying to get in. Plus, when the scientist notice on their computers that there's an increase in mass, one of them simply assumes it's a bird and then they continue their experiment. Instead of making assumptions, they should have halted their experiment and have someone take a look to see what caused the increase of mass.
Stupidity: Ashley Judd finally tracks her husband down and what does she do? She doesn't try to capture him to prove her innocence, she doesn't notify the police, she doesn't take his picture and send it to every police dept in the US, she doesn't even follow him to find out where her son is. Instead all she does is ask for her kid back. What was she going to do even if she got him back? She is still on the run from the law, with little money and resources and she doesn't even know if her kid even remembers her.
Stupidity: John, the guy who taught Chucky the voodoo spell is confronted by Chucky later on in doll form. When things don't go how Chucky wants, he finds a Voodoo doll of the same guy just lying around and uses it to torture him. So this idiot who knows Voodoo and what it does, happens to have a Voodoo doll of himself just lying around? Yeah sure, that can't go wrong at all.
Stupidity: When Cathy, Chris and Carrie go into their late grandfather's room, they discover a copy of the grandfather's will that states that if it was ever discovered that Corrine had children, she would lose everything. Had they taken the will to their mom's wedding and showed it to her fiance, they would have been able to expose the scam much quicker. Instead, they take the dead mouse Fred with them and show it to everybody. What good would a dead mouse do when a will has more leverage?
Stupidity: After Josh runs away from home after being turned into an adult, nothing seems to be done about his "disappearance." The only indication that he is missing is his picture on the back of a milk carton. There's no flyers about him missing. No mention of his disappearance in any newspapers or TV news about his mom saying that he's been "abducted" by a stranger. Even when Josh writes a letter home, he uses the actual address of where he's staying, but no cops turn up at his door after his mom receives the letter.
Stupidity: The Death Star comes equipped with a powerful tractor beam capable of capturing a ship the size and agility of the Millennium Falcon. Why don't they use it against the rebel fighters attacking them at the end of the film? Okay, Obi-Wan Kenobi turned it off earlier but I find it hard to believe that someone who has never before visited the largest, most complex space station in the Universe and who was previously unaware of its very existence can disable a fundamental security system but the people who designed, built and run the whole thing can't work out how to switch it back on. They should have no problems with this, considering the fact that Obi-Wan didn't damage it.
Stupidity: Steve Trevor is the leader of a group of Allied spies, and they are traveling with a gorgeous woman who has shown to have amazing fighting skills and super powers, but they allow a group photo to be taken with Diana in her armor. They don't know who is taking the photograph nor what they intend to do with it. Their photograph circulating through a news publication or passed on to an enemy's intelligence service could compromise himself and the rest of his team, and they would absolutely know that. Undercover agents do not pose for photographs.
Stupidity: In the scene where Billy, Stu and Sidney are in the kitchen (when Sidney finds out they are the real killers), they begin to stab each other as a part of the plan to fool the police into thinking they were almost killed by Sidney's father, whom they were going to frame for the murders. They were planning on killing Sidney and her father, so it makes absolutely no sense that they begin stabbing each other BEFORE killing Sidney and her father, especially since Sidney would be at more of an advantage of outrunning them should she have the opportunity to since they have both been stabbed multiple times.
Stupidity: When Anna, Velkan and the villagers successfully trap the gray werewolf in a huge cage, the villagers start firing on it. Anna says they need to use Velkan's gun because it has silver bullets in it. Since everybody knew they were fighting a werewolf, all of them should have had silver bullets for their guns.
Stupidity: The climax takes place during the launch of the "Buddi 2," a hotly anticipated tech gadget. The entire film has been leading up this point, and it's a big deal that it's being launched. And yet, there are no more than maybe 20 people waiting. Not a mistake per se, but totally unrealistic compared to the huge crowds these sort-of launches typically bring in.
Stupidity: When Lawrence begins turning into a werewolf in front of Dr. Hoenneger, his fellow doctors, Inspector Aberline and the guard, everybody just sits in their seats and watches the transformation instead of running for the door.
Stupidity: When Charley goes to Evil's house, he asks him how to protect himself from vampires. Since Charley is such a huge fan of Fright Night and watches it all the time, he should already know everything to protect himself from Jerry.