Stupidity: In the freezer you can see the electrical wire power cord that is powering the motor on the wall. She could have easily pulled/ ripped the wire to stop the freezer from working. And when the chemical bomb is in the mixer they could have unplugged the machine which would stop it from going off.
Stupidity: When the birds gather in the schoolyard, Melanie runs in to warn Annie. Rather than calling the police or fire department for help and then shelter in place until aid arrives (which would take about ten minutes), they instead decide to make a run for it with the children, out in the open, getting attacked by the birds. As it's the 1960s, the schoolhouse presumably has a working phone.
Stupidity: Liv and her five friends went to Willy's to set it on fire, but Liv insisted they first go inside to get the Janitor out. All were aware of the situation and danger related to the mass murderer and the animatronics, but Bobby and Kathy went into another room to "hook up." Kathy recognized the room as the one where the killers committed suicide and later saw movement, but she continued having sex with Bobby until he was attacked and killed. (00:38:48 - 00:42:02)
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: During the final battle, Gomm kills one of the rednecks by sticking his tentacles into the man's eyes. However, the entire scene is shot very sloppily. The man has his gun trained on Gomm for almost 10 full seconds, and just stands there while the tentacles come out slowly, Gomm laughs, and then the tentacles shoot forward. Given the amount of mayhem and creatures, there's no reason for the redneck to have not just shot Gomm during that period of time.
Stupidity: All that chaos going on at the end of the movie. Gun shots. Screaming. Blowing up police cars. Yet not one neighbor comes out to see what's going on.
Suggested correction: Gunshots and explosions going off at a neighbor's house isn't going to make me go and investigate. It's going to make me call the police and hide in my own house.
Stupidity: When Bobbi Jo sees Ash's severed possessed hand holding her hand, she freaks out and runs out of the cabin and then gets killed by possessed trees. While it's certainly natural for her to be terrified, why would she run AWAY from where all her friends are and go off by herself?
Suggested correction: She is a civilian in a blind panic, and even people with actual training have done dumber in panics (e.g., choking firefighters run away from other firefighters who could help them). She also has no idea how dangerous the woods are at this point; she walked to the cabin just fine and only saw scary things inside it, so she thinks she'll be fine if she just runs from the cabin.
Stupidity: When Jody traps Michael in his room, he does so by sticking a screwdriver between the the door and the jamb. This likely wouldn't accomplish much regardless, but it especially wouldn't work in this situation because the door opens into the room, so Michael could simply open the door and the screwdriver would fall on the ground.
Stupidity: Considering the movie takes place in the winter of '75, it seems unlikely that the babysitter would arrive in a halter top and barefeet.
Suggested correction: I agree that this is a "stupidity", but think your use of the words "it seems unlikely" allows for the suggested corrections already given. Perhaps replace "it seems unlikely" to "it isn't rational." Even if the daytime temperature was mild for winter, the temperature would probably drop by around 30° overnight.
Suggested correction: Actually, January of 1975 was very mild, much like the winter in the northeast this year, so it would not be impossible for the babysitter to wear unseasonable clothing.
Suggested correction: Ordinarily I would agree with this observation, however, I believe the girls outfit matches her carefree attitude. She did bong hits in the family bathroom, laid on the boys bed seductively and asked him if he French kissed so dressing comfortably to babysit isn't out of context.
Adding to this, almost every babysitter I had growing up went barefoot or at least shoeless. Even this past new years eve, the young lady who came to watch my kids took her shoes off as soon as she came in the house.
It is a common courtesy to remove shoes when entering someone's home to protect the carpeting (or other flooring) from getting soiled by whatever might be on the bottom of shoes after walking outdoors.
Maybe I'm wrong, but after watching this movie last night, it sounded like the girl was wearing flip-flops when she walked across the bathroom to open the door. Would it be rational for her to have worn sandals too?
Considering she was barefoot for most of the night, yes, she may have worn them for comfort.
Stupidity: Charlie set her mom's blouse sleeves on fire and mom/Vicky just stood in the same spot, holding her arms (burning sleeves) out in front of her until Andy came and smothered the flames with a blanket. Even children know to "drop and roll" or put fire out by smothering it. (00:22:56)
Stupidity: The whole movie happens for unbelievably convoluted causes. Despite their friendship and the simple fact that he still runs an occult shop (showing that he's not exactly insensitive to the past), Ray in years never spent a single moment to check on Egon's whereabouts, which he knew, or listen to him. The city has Shandor's name all over and any cursory investigation would have unveiled the connection. Assuming of course that Egon turned into such a lunatic he couldn't do the namedropping himself.
Stupidity: The chef/cook of the house has his rifle, and he knows Morgan lost control and pretty much killed everybody on the premises. He comes face to face with her, but he lowers his rifle instead of taking the subject (Morgan) out with a shot. There is no logic here but to only pro-long the film. There's no reasoning with a scientifically created subject which is killing everybody it comes into contact with. (01:09:00)
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: The cleaning crew can't open doors and stand in front of a large window. They see a couple walking a dog, so they scream, "Hey!" and wave their hands in the air, even though the people are not looking their way. They do not bang on the glass or use their flashlights to try to draw attention. (00:29:13)
Stupidity: Officer Malloy was trying to fix reception problems on a small TV at the police station while Lt. Stevenson was listening to Gerri Tauver reporting on the explosion that happened in Suffolk County. Stevenson told Malloy, "Just play with the wire... I need to fix this. I need to find out what's going on from the explosion this morning." A police lieutenant could have easily called the fire department chief or officers on the scene to get first-hand information, not depend on a TV news report. (00:09:09 - 00:10:54)
Stupidity: All those people at the bus station yet no-one reports a blood trail from the phone to the bathroom? There's even blood in the sink.
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 one of Vincent Price's protege's leaves a wax work-in-progress, he doesn't cover it with a lightly damp cloth, which is very important: wax uncovered will go hard and useless.