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: Mason should have also removed the other gun he had in his office for display, or he could have removed the bullets from the guns. There is no reason he would have to use these guns for his victims, so it makes no sense to leave the other gun in his office or have them loaded with bullets.
Stupidity: If Leo wants to kill his son's killer at any cost and no matter what happens, he should've gone directly to his House when the purge starts.
Suggested correction: He absolutely is headed there, but we see at least one incident where his path is blocked due to people out purging, and he needs to turn around. That detour caused him to go a different route, which led him to the mother and daughter. We even see him struggling with the decision on whether or not to help them. He decides to do so, and the rest of the movie unfolds as we see, but there is no mistake.
Stupidity: Near the end, Sydney Wells and Paul Faulkner try to rescue a little 10-year-old girl who is trapped in the recreational vehicle. Sydney is banging on the door from the outside while the little girl is banging on the door from the inside. Paul then smashes out the door window, reaches inside and easily releases the door latch in one move. So, a 10-year-old girl was "trapped" because she couldn't simply open a door latch? Stupid.
Stupidity: Two oil rig workers were looking at two young ladies in bikinis who were sunning themselves on a boat/yacht. One oil rig worker said, "That's where we should be right now. With them, enjoying the sun, instead of baking in it." Both men had double layers on - a jacket or sweater over a jersey or T-shirt - and one man was wearing a tossel cap. Even if it gets cold on the rigs at night, they did not need to be dressed in warm attire during the sunny daylight hours. (00:10:35)
Stupidity: On the phone, Marion told Paul, "I'm nearly certain she'll [Maggie] get full custody, but if you fight this, you stand to lose everything, even visitation to see your daughter. Sign the contract..." Jeff wanted to see Jenny and, other than being unemployed for three years, there were no reasons his parental rights should be terminated. It was not "in the best interests" of Jenny to relocate to Seattle and never see her father again. Fighting it, Jeff would have likely won custody of Jenny. (00:15:39)
Stupidity: A zombie is climbing up the apartment wall and is almost at Aidan's balcony railing. Instead of trying to push the Zombie off while he had the advantage (and there was a surfboard on the balcony that could have been used), Aidan ran inside and locked the door to the balcony. This furthered the plot and added excitement by having zombies coming at Aidan from multiple directions. (00:43:09)
Stupidity: Why would they drive all the way to Marylebone Railway Station in central London when Southbury Road station is only a thirty-second drive away?
Stupidity: When Joey is trying to escape from the Cenobites, she bumps into a man who calls her "Baby" and wonders where she's off to in such a hurry, possibly because he's looking for a booty call. The streets are literally on fire (there's a huge flame right next to the man) and buildings are exploding. Even if he's extremely horny, he should still be able to tell a woman doesn't want to stay around with so much chaos and destruction happening around them.
Suggested correction: The dude was probably high on various types of drugs and probably didn't even know where he was. Just a thought.
Too high to recognize buildings exploding and fires bursting from out of nowhere, but not too high to recognize a woman in distress?
He didn't recognize much did he? Well, he saw she was female. But not really what was going on.
He recognized she was off somewhere in a hurry. He also had a pretty instant reaction to seeing the Camerahead Cenobite.
Stupidity: *SPOILER* Cecilia does not want James (or anyone else) to know she uses/wears the "invisibility suit", which, recorded, makes it look like Adrian slits his own throat. Cecilia leaves the invisibility suit in plain view near the top of her open purse/bag, where James notices it and knows she kills Adrian. (01:51:35)
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: When Olivia said, "Momma. It's hot", she was wearing a long-sleeved shirt over her overalls and shirt. Her mom, wearing a sweater over a tunic top, replied, "I know, baby" and asked Steve to turn the A/C on in the RV, but the A/C didn't work. Instead of removing some layers, Steve opened some windows. Jay complained how hot it was, but he wore a hooded sleeveless sweatshirt over his T-shirt (and never took it off, even when outside in the desert). (00:09:31 - 00:46:00)
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: 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)