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?
Suggested correction: We never see them put the will away. It's possible one of them had it in their pocket during the wedding scene, but they just never showed it because A. Showing the will would have given motive for their mother to want to kill them, but not have proved that these children were hers, and B. The dead mouse is a more visceral, hate-fueled move that matches Kristy's anger.
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: Given Laurie Strode survived an attack by a still on the loose maniac she's not guarded at the hospital?
Stupidity: Ghostface ambushes Sidney and her friend with the two detectives protecting her while in the car. Ghostface busts out of the driver window, and slashes the throat of the driver detective and then assaults his partner, kicking the crap out of him. Ghostface is obviously going with the intent to kill and has a weapon. He just killed one of the detectives and threw the other in the street in front of the car. When Ghostface jumps in the car to take off, the living detective, beaten, jumps up with his gun trained on Ghostface. But rather than do anything smart, he just yells at the killer, the cop killer, to freeze and get out of the running car that he's standing in front of. He just saw the guy kill his own partner and knows he is intending to kill the girls in the back. His gun is pointed at Ghostface who is about to drive over him, but he just yells at him instead of shooting.
Stupidity: You'd think that Jezzie would have been a little smarter as a secret service agent. You know, like once they acquire Megan, to delete all the incriminating evidence sitting back on an unattended computer. Aside from the fact she also used quite a stupid password, a single password isn't very secure at all. And then the content, displayed instantly when you get in, might as well say things like "hey look, we know what the kidnapper looks like" and "wow, here's the boat he's in too" and "wow, a cabin for sale. Why don't we go hang out there with all the evidence." Come on....they might as well have left a trail of breadcrumbs, because they sure were asking to get caught.
Stupidity: When Ruth catches Chuck fishing for turds in their toilet, he yells at her and asks for privacy and then slams the bathroom door shut. He should have closed the bathroom door in the first place if he didn't want anybody to see what he was doing. It's not like anybody would think he was weird for going into the bathroom and immediately closing the door.
Stupidity: Jill wasn't thinking too clearly when she plans her story. She only wiped the fingerprints off the gun but they would be all over the tape on Trevor's body. Then she wipes off the knife yet touches it again to slide it across the floor. She was thinking that Trevor and Charlie were going to be thought of as the killers but their fingerprints weren't on either of the weapons.
Stupidity: When the teens are about dump Ben Willis off the pier, he regains consciousness and snatches Helen's crown. Despite knowing he's alive with strength to spare, they nonetheless drop him in the water and continually refer to him as "dead."
Stupidity: Marta is shown to barf whenever she lies, and Blanc knows this. He should've asked her who did it right away. The entire plot of this movie is rendered unnecessary.
Suggested correction: That would only work if Blanc knew that Marta had knowledge of who killed Harland and if he came to the conclusion that Harland actually killed himself. Blanc doesn't know that Harland killed himself to protect Marta until Marta confesses. Also, this wouldn't have solved the "who done it" anyway, since Marta had no idea that Ransom switched the vials.
Stupidity: When the parents knew Freddy was doing the killing then why didn't they watch him like a neighborhood watch so they could catch him trying it again, and thus avoid the legal technicality which threw out of the first case? It seems idiotic they broke the law to punish this lawbreaker.
Suggested correction: The parents believed Freddy would not face justice for his crimes. They were unwilling to allow him the opportunity to kill another child, and believing the justice system had failed them they took the law into their own hands. They completely got away with their vigilante justice. The only bad thing that happened was Freddy was brought back as a vengeful demon with superpowers. There is no way the parents could have known this would happen.
It is still a stupidity. Even if they wanted him to suffer, they would try to avoid legal problems at all costs. It is good that they wanted to prevent him from killing another child but they still shouldn't act so hastily.
Stupidity entries are not meant to be stupid acts by characters. They're for minor plot holes. Without in-film evidence why the acts would be a plot hole, there is no stupidity mistake. And assuming what characters would or would not do does not make a valid mistake (not to mention revenge killings do happen in real life and in movies a lot).
No, it's not. What else were they supposed to do when he was caught and set free? It's still not their fault what happened next.
Stupidity: (SPOILERS) Despite knowing that Rowena killed the last person who was blackmailing her, Miles tries to use her secret to blackmail her into sleeping with him. Did he really believe that she wouldn't react with such drastic measures like she did before?
Stupidity: Oldboy finds the right Blue Dragon restaurant that the prison uses. But him running after the delivery boy doesn't make sense. How does he know the delivery boy is going to the prison? He knows it's the right takeaway, but that's all he knows. So, would he have a moped, bicycle, or car ready to use surely? The takeaway is a long way from the prison (as he says himself in narration), so he wouldn't just wait around for a moped delivery to run after, especially as a good chance he wouldn't be able to keep up.
Stupidity: When Roy finally gets to the Lima project in the shuttle but can't dock, he just lets it drift away - he could have at least tethered it to the station to use it for his return trip.
Suggested correction: There is no way to know the reason why he did this. It's plausible, or even likely, that the pod was out of fuel. Not only that, but it could not dock with Lima because it was damaged, so it's likely that the damage played into the decision as well.
Stupidity: The protagonist with her kid stops a car with two guys, in the middle of the night in an isolated country road. Both the adult and the kid scream that a dangerous maniac is chasing them. So naturally, the two guys (who do believe them, it's not that they are being dismissive) offer them some hot coffee, and they drink it right there in the middle of the street, not dialling for help, getting in the car to drive away or anything, because that's what normal people do when you tell them a serial killer is nearby; stop for a hot cuppa.
Stupidity: When the rebel leader talks to Rick Flag guaranteeing her help, she mentions that she will help to find "this Gaius Grieves", speaking as if it was someone unknown to her. He's been around the island (going to bars and surely not being reclusive) for 30 years; it's pretty difficult to think it would not be one of the most known people on the island, given his role, unique appearance and the small community.
Stupidity: The security system is able to automatically ascertain the malfunction to the exact valve, and the electrical problem, but the alarm is not triggered at that point, but only when the holding cells are already filled with water and Breslin is making his return. They are on a ship, the fact that it's taking on water feels like a pretty important issue to monitor.
Stupidity: After Danny kills Kat on the sidewalk, he just stands there with Rory while the other gang members flee to their vehicles. William comes running out of the Mexican restaurant and Rory tells Danny, "We gotta go, brother. Now." Rory sees William and gets into the vehicle without getting Danny to go with him. The gang drives away with Rory watching out the window as William tackles and fights with Danny. Rory and the rest of the violent gang left behind someone who just passed the initiation. (00:07:10 - 00:07:40)
Suggested correction: As you said, she's on the run from the law. Why would she call the cops and risk going back to prison and losing her son again? Even if her husband was alive, she has no way of knowing what will happen to her. This was her best shot (especially in her mind) to getting her son back, thinking her husband would rather give up the boy's location rather than die; whereas height keep the boy's location a secret if all he was facing was possible jail time, if that.