Stupidity: The marble somehow got under Don's skin at the bottom of his leg and was moving up the length of his body. Instead of heading off the marble in order to intercept and stop it from continuing up his leg, Don got a knife/box cutter and jabbed it into his skin near the bottom of his leg and trailed the marble, causing a long gash that served no purpose (other than creating a lot of blood and making a gory scene). The marble eventually exited his body via his eye. (01:04:40)
Stupidity: The babies could have simply been swapped at the start. There was no reason Father Spiletto needed to inform Robert Thorn of his son's death and then convince him to take a different child instead.
Suggested correction: Perhaps, but then again the Satanists were trying to cover all bases. If they simply swapped them, the parents may start questioning things when Damien grew up and they might have noticed that he didn't look or behave like either of them. For insurance they could have brought Robert Thorn into the plot (without his knowledge of the full story), so that if she asks any questions he could simply stick to the story, offering a cover to the Satanists. During the conversation, he mentioned that the baby dying would be devastating to his wife (which is why he agreed to the switch) so his intentions were pure, even though it ends badly and tragically for both him and his wife.
Stupidity: Throughout the entire movie the police just don't seem to be doing their job. From the beginning they never taped off the RV as a possible crime scene, for 4 days every night the police always have their cherries on, and they are standing in front if the lake looking in the same spot about 50 yards away from the RV. They never searched the buildings on the grounds; which the mother searched and found the sicko responsible for making filthy movies. They also never searched the trash. Doesn't jive. (00:00:01 - 01:50:55)
Stupidity: When Poirot's team attacks the Germans, they do it through the cover of gas they release. The Germans are having a good time chatting and not taking the situation seriously when there's a big cloud of noxious gas advancing towards them. Despite Poirot's oh-so-clever plan, they should by all means know that something is coming their way, but they get caught entirely with their pants down.
Stupidity: Sherlock Holmes, a "simple" detective, knows the whole story of an English officer and that of a Nazi secret agent and their great physical resemblance, but Barham's colleagues in British Intelligence do not know any of these facts and are surprised when Holmes tells them all the details.
Stupidity: For Sarah's murder to work, a lot of coincidences and unpredictable, convenient behaviour from the victim have to happen; she has to Miss the security guard and not hear them, she has to seek shelter of all places right in the costume room full of effigies of the killer, where the killer himself happens to be posing, she has to be completely inefficient at calling help with her cell phone.
Stupidity: When Sook-Hee and Hideko are leaving the mansion to meet up with Fujiwara, they run past a rope and noose tied to a tree. Later on, it's revealed that this rope was tied to the tree because Hideko attempted to hang herself. Despite abandoning the suicide attempt and coming up with a new plan to scam Fujiwara, they leave the rope tied to the tree, even though this could potentially expose the scam to someone at the mansion. (00:53:32 - 01:47:15)
Stupidity: Cobb convinces Fischer to enter a dream saying it's Browning's dream. A couple of scenes later he says it's Eames' dream, in front of Fischer, who doesn't know Eames was impersonating Browning. (00:51:35)
Suggested correction: Firstly, the Browning that the team "catches" is not Eames, it is a projection of Fischer's subconscious. Eames was only impersonating Browning in the first level. Secondly, Cobb doesn't say they are going into Browning's dream, he says they should put him under and do to him what the kidnappers were planning to do to Fischer. That is, use Fischer as the subject of the next dream level. Fischer knows that Eames is the dreamer on the third level, he even jokingly asks why Eames couldn't dream of a beach. But he thinks that Eames is part of Mr. Charles' security team. He believes that Browning is to be the subject on the last level when in fact, Fischer is the subject on each level. Because of how effective the Mr. Charles gambit was, Fischer is unaware that the team has been fighting his subconscious all along.
Stupidity: The protagonist knocks at every door, humiliates himself, losing any hope of getting his dream job, jumps through all sorts of hoops to get a court order to save Crawford's wife, but somehow, once he gets it, he has to drive through LA and physically get to the hospital room where euthanasia will be performed, because he couldn't be bothered to phone the hospital to tell them about the court order or talk to someone in the lobby.
Stupidity: I seriously doubt that any Mexican Mafia cartel operating here in the U.S. would waste their time eliminating two simple patrol officers. There's nothing to gain by doing so, and it would bring a war between themselves and the police, which in return would bring hundreds of arrests of key members, drug busts, raids of their operations within Southern Cali, etc. They wouldn't risk their business taking millions of loss of revenue all to kill two patrol beat cops.
Stupidity: When the police turn up the first time the mother lies to them to get them to go away but the burglars can't hear the conversation. She could have told them exactly what was happening and that she had to make it look like she'd got rid of them. She could have then smashed all the cameras and just let them in.
Suggested correction: This was a character decision, and a completely understandable one. Her daughter was being held hostage in the panic room. She decided to cooperate with their demands for the safety of her daughter. Your idea is a solid one, but she simply did not think of it in her state of mind. She opted to get rid of the police, and her decision is not a movie mistake.
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: When White and Exley meet outside the motel and realise they've been set up, White suggests they leave, but Exley says it's too late. It wasn't even close to too late. There were no other people around, they had cars and there was a road right in front of them. Then they decide to go trap themselves inside the motel. It seems like they wanted to be shot.
Stupidity: Ilona Bergen's murder happening as shown is laughable; she sees the hand with the syringe coming, slowly, at her, and not only she can't defend herself or avoid it (because, damsels in distress in the 60s, you know), but she does not even scream. There's no way the killer could have been sure, or even take a calculated risk, to murder her in such a way. (00:55:25)
Stupidity: The ending has a bit of a twist; apparently, octogenarian college professor Morgan Freeman somehow chased down and murdered offscreen 6'3" professional assassin Vernon Davis. That's not the toughest part to swallow, though; he also sent the two detectives (one of whom he is not even supposed to know of) via UPS 2-day delivery mail some body parts of his victim. We don't know which part Lavazzi gets (genitalia were mentioned earlier), but Boyd gets the eyes and decides to eat them. For starters, unrefrigerated 2-3 day old eyes would never look as pristine as the ones Boyd gets. Also, Morgan Freeman mentioned earlier in the movie that for the power of Muti to be effective, the body parts need to be taken from a victim that is alive, screaming, and eaten only mixed with herbs and other catalysts. So what the detective does doesn't make sense even in the movie lore.