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 Once the cash is withdrawn, the card is thrown away, and nobody is at risk.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: 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: 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: 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 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.
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: Someone with the Punisher's tactical knowledge would not hang upside down from a chandelier to shoot in a 360-degree pattern at his targets. This would make him a sitting duck and it's only because of the movie tropes of Stormtrooper aim and cannon fodder henchmen that he isn't killed because of it.
Stupidity: If the song "Beyond the Sea" can be continuously broadcast on the radio station as a "cryptic clue" that there are survivors living on the island and the aliens are unable to swim, then someone could verbally announce that information over the radio and otherwise communicate with other humans. The same thing about how Regan's hearing aid incapacitates the creatures. Rather than explaining how to weaponized the high-frequency sound, only the hearing aid's noise is broadcast and anyone hearing it has to figure it out.
Stupidity: At the end of the film, the Rehabs recruit gang members and give them all kinds of guns from shotguns, handguns, and automatic weapons. The cops and deputized citizens have similar weapons. But for all the shooting, not one gang member is hit and only one citizen appears to be shot (aside from the three blown off the building roof by the Rehab tank.) I don't think anyone has any bullets.
Stupidity: The entire plot hinges on a military facility that knows it's housing what's essentially a sentient world-ending disease, and that has the foresight and resources to build a containment room with a cyanide failsafe, having nothing else to stop said virus from breaking out beside the glass her room is mostly composed of. Once she's out of that room, she just has to run through an unlocked, labeled exit door and hop a fence. Where are the emergency bulkheads? Where are the armed guards watching the hallways? Why isn't that fence electrified? Why isn't she wearing a metal shock-collar at all times? Even if that glass was bulletproof and/or they seriously underestimated her strength, they'd still have to open the door to her room at various times to give and take her food, books, laundry and waste.
Stupidity: After the team break Solomon Lane out of custody, they drive around Paris with him in the front seat of their car. Lane is still wearing a straightjacket, bound in chains and has a black bag over his head. The IMF team drive him through crowded streets, past multiple police cars in full view of everyone just minutes after an armoured convoy transporting a dangerous prisoner (who everyone is now looking for) has been attacked. None of the Parisian police seem to notice this and none of the IMF team questions the tactical soundness of this approach.
Stupidity: It's stated that Imhotep will fear cats until he has fully regenerated and two different scenes show him fleeing in terror at the sight of a cat. Despite this, none of the characters that Imhotep is trying to kill that are fully aware of his weakness even think to have a cat with them at all times.
Stupidity: At the prison yard, when Weiss overhears that the sonic deadline was being deactivated to let new prisoners in, he manages to look at the deactivation code. The guard sees Weiss looking at the code and switches the computer off. If he had changed the code and informed another guard, none of the prisoners would have escaped.