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.
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 murder during the opening scene is ridiculous. Angela chases a screaming woman down an alleyway, in broad daylight, in front of crowds of witnesses, while driving a truck, runs her over, then throws her body in the back of the truck and then waits right next to the alley where she murdered the girl to be picked up by the van headed to the camp. At no point did any one of the numerous people on the street try to help the girl or even just investigate the alley to see if she was hurt (supposing it was an accident) or just call the police?
Stupidity: Nicholas Cage saves the kids coming out of the dark dimension exactly at midnight but gets back home at dawn. It should be noted that he has two other kids to bring back home and if we assume he did that already, it would explain part of the time discrepancy. In that case he kept his wife up all night when she was fearing for the safety of both without a word, but she's not upset or anything.
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: 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: During the movie, the bad guys keep the main character alive and free to roam the facility at will (even if they made him sign an incriminating form at the very beginning stating he's an inmate) doing absolutely nothing to restrict his freedom till the very end - he even retains personal effects like his broken Rolex and lighter, his wallet full of cash! He breaks into every forbidden area, picks up fights, damages property, escapes multiple times and 'corrupts' the person the whole facility is built for. He is worth absolutely nothing to them and has nobody waiting for him or that will look for him.
Stupidity: When Michael is heading to the hospital and he just learns that the Cloverfield Station is gone, the whole scene of him driving is super unreal. It looks like he's not even driving, like it's a self-driving car or something because the steering wheel doesn't move at all while he's driving, and he isn't looking at the road (staring into space). The whole scene just doesn't feel realistic. (00:43:48 - 00:44:30)
Stupidity: During the ghost chase montage, why would Velma look for her glasses with her hand in midair instead than on the ground? No matter how nearsighted you are, the closest wall, or obstacle, is meters away and you'll never find your glasses anywhere but the floor. She does it for a longer time than anyone would. (00:52:40)
Stupidity: The marines all wear helmets as part of their protective gear when they go into the Phobos compound, but Bennett and Akua do not wear any helmets at all. They wear bullet proof vests and arm themselves with weapons, but unlike the other marines, they go in without any way of protecting their heads.
Stupidity: Quinn wasn't able to kill Dr. Sullivan, so decided to kill herself to save her sister (by dying earlier and ending the curse). Instead of telling her younger (teenaged) sister about the Narcan and how to use it, she writes Narcan on her upper arm, assuming her sister will see it in time, know that there is a syringe in her pocket, and not hesitate to inject the Narcan. Quinn left too many steps up to chance and did not take the necessary precautions to help insure she could be revived. At the same time, a registered nurse - even a recent graduate - should know that this would not be the circumstances under which Narcan might be successful in counteracting the morphine. Taking time to say, "I love you, Jordan. I'll always be with you" was wasted time that should have been used giving her sister instructions because Quinn (erroneously) believed her plan would work. (01:17:08)
Stupidity: Crystal knows she is being hunted and in a very dangerous life-threatening situation yet, after she kills "Ma and Pops" in their Main Street Market, she grabs only a few shotgun shells out of the open box instead of taking all of them or as many as she could find. Also, she walks some distance to the store but did not take anything to drink or eat. (00:27:00 - 00:27:55)
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)