Stupidity: In the opening scene, Sonny has come to her now-deceased father's house. Police and the coroner are in the basement taking notes while standing around the body - which is face down and awkwardly lying at the bottom of the steps. Det. Sabatino informed Sonny that the death was ruled accidental due to a high blood-alcohol level. Detectives or the coroner did not rule out other possibilities: natural cause, suicide, or murder. High blood-alcohol level could be secondary.
Stupidity: Possessed child Nicholas made the cross from a necklace pierce Fr Louis in the neck, causing blood to gush out. Fr Peter yelled, "Fr Louis, Fr Louis" as Fr Louis tried to pull out the cross. No one else present rushed over to try to help stop the bleeding or assist him. Fr Louis' sash could have been used around his neck to apply direct pressure to the wound. (Although it might have been obvious that it was a fatal wound to the neck, someone should have at least tried to help). (00:07:15)
Stupidity: Judy Hicks, a trained cop and sheriff with over a decade of experience, does not carry a back-up holstered gun on her when she is off-duty, one day after her son's close friend is stabbed seven times near to death. A trained police officer/sheriff would also question why the killer would tell her they are about to kill her son, before they do it, if not to bait them and lure them into a trap. (00:45:58 - 00:46:30)
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)
Stupidity: When Amy heard Luce coming toward the kitchen, she "hid" his school paper and bag of fireworks in a kitchen cupboard, apparently while knowing Luce would be doing the dishes (and putting them away in the cupboards). Amy and Peter were on the patio (with the door open) just outside the kitchen when Luce was doing the dishes yet Amy made no attempt to prevent Luce from finding his paper and bag of fireworks (which he did). (00:16:20 - 00:21:00)
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.
Stupidity: Simi sets the wooden tepee structure on fire with Claudia next to it, and Claudia's clothes get engulfed in flames. Claudia continues to stand next to the fire, flailing her arms instead of following the commonly-known "learn not to burn" procedure that is taught to children: "Stop, drop, and roll" to extinguish the fire on one's clothes - and to stay away from fire. (01:31:00)
Stupidity: Stephanie googles (using the actual Google engine) the name of Emily's fiancé but gets zero results. He is the heir of a mob family embroiled in decades-long feuds, a central figure in the Roman nightlife travelling with private jets, with the newspapers having covered their earlier relationship. Therefore, the chances that absolutely nothing exists online are exactly zero. (00:19:05)
Stupidity: Alex is the only child that did not go missing. No one has seen his parents, and he is presumably still at risk, but neither the police, social workers, nor the school think to do a home wellness check, where they would have discovered the newspapers covering the windows.
Stupidity: Despite being assaulted by an intruder that she managed to temporarily fend off, the female neighbor takes forever to say something when she calls the police, which is just enough time for the villain to regain his senses and attack her with a huge drill.
Stupidity: After Jack is found not guilty, you would think the first thing he would do is to get rid of the typewriter he used to send the notes to Teddy to manipulate her into winning the case for him, especially when she is sleeping with him at his home. Not to even mention the fact he hid the typewriter in an unsecure manner in a closet behind a stack of towels, a place where it would very easily be found. This foolish mistake exposed the truth to Teddy and cost him his "freedom."
Stupidity: The first time they see the bad guy in the Land Rover, Morgan has him in his sights but can't shoot, as he doesn't want to kill him. Clearly, the obvious thing to do is to shoot the car tires, but he doesn't and lets him drive off.
Suggested correction: Shooting out car tires is not an actual effective strategy to get a vehicle to stop. A 9mm bullet simply doesn't cause enough damage to a tire to get it to go flat immediately. It causes similar damage as running over a nail. It is a movie cliche, similar to using a gun to shoot out a lock or a rope, it just doesn't happen in real life.
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: When questioning the heavyset witness after his having seen Costner in the hallway, there is no mention of the most obvious identifiable feature...the pearly white Navy uniform he was wearing. Instead, Costner is described as being a common "everyman."
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.