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: 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: I won't begrudge Father Lamont for instinctively grabbing for the first thing in reach to try to smother the box on fire (even if it is a pair of wooden crutches), but you'd think at some point he'd notice that he's just spreading the fire and making it much, much worse, but he keeps at it until the doctor finally returns with the extinguisher.
Stupidity: The location of the small hole near the bottom of the wood-trimmed wall where Everette passed Sharyn a note and pencil would have taken much more time and effort to accomplish compared to merely penetrating the wall a little higher (where there would not have been wood trimming/baseboard on both sides). (00:10:00)
Stupidity: Ben passes out on the living room floor. Before Jack leaves, he places a lantern-type flashlight a couple of inches above Ben's right shoulder. Apparently, Ben's first move when he wakes up in total darkness is to move his right hand from alongside his body to behind his shoulder, where he just happens to find the flashlight. Even if Ben's first reaction is to feel for a head injury, his hand does not go high enough to bump into the flashlight. (00:02:12 - 00:02:40)
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: 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: Dr. Emma Collins and Shaw were in the water with Sally the Great White Shark. Emma put her right hand over Sally's nose and pushed her back. Sally swam forward and Emma pointed her index finger at Sally's nose then pushed her back again. Emma told Shaw, "If you stay calm and redirect" and "Don't try this at home" before Sally pushed Emma with her nose on Emma's stomach and rammed her into an underwater object. Emma was the instigator, goading a Great White Shark. Emma could have been killed. (00:05:10)
Stupidity: After Caleb escapes the vampire "family" with his father and sister, he undergoes a blood transfusion which cures his vampirism. As soon as this was done Caleb, his father and sister should have fled somewhere else immediately. The vampire "family" knew where he lived as they kidnapped him in front of his home. Instead, they stick around the home, with no obvious intention of leaving. Of course, the vampires track them down and kidnap Sarah.
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: Lyra says, "We can't call the police. We broke into our neighbour's house. There's a dead body downstairs and there's an alien running around." Given that everything can be attributed to the alien, the police do not know (or have to be told) the friends broke into the house or that they "weren't supposed to be there" - they can merely say they heard loud noises. The friends stay in the house, hiding and fleeing from the alien. (00:32:47)
Stupidity: If there was a live camera feed right where the eggs were, one wonders how everyone missed the 'small' fact that they hatched. That of course combined with the fact that going for no reason in the nest of the creatures that nearly killed another crewmember is suicidal, and Bob goes with no weapons and with one small plastic fruit crate to fetch eggs that are bigger than an ostrich's. (00:55:25)
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.