Stupidity: Arishem's plan with the Deviants hardly makes any sense; he sent out biological beasts that he can't control, to kill off the natural predators of every planet. Forgetting the fact that obviously it didn't work (but the movie does not say that) because Earth has always had predators, that's a terrible plan to begin with, since any ecosystem needs predators or the other animals will grow uncontrollably.
Stupidity: When the crew think that Dorothy is a robot with a bomb planted in her and is capable of exploding, they begin to think that she is about to explode and attempt to dodge it. Rather than hide somewhere or take cover behind an area that may protect them, they just get down on the floor in front of them and cover their heads with their hands. If they thought the explosion was going to be as deadly as the one they saw on the news, this method of taking cover wouldn't be even remotely effective. (00:24:40)
Stupidity: Gunner [Director and Co-Writer Brock Harris] was given a puppy ("Barnaby") by the nurse (?) who told him he was sorry his "[VA] benefits didn't go through." Gunner could have merely refused to take the puppy but, instead, drove away with it - and later threw it out his truck's window while he was driving. (00:24:42 - 00:25:55)
Stupidity: Why would the tow driver go around Deacon's truck in the one-way dead-end parking lot to pick it up from the front? He'd have to back it up on the hook, and Deacon's truck would be in Park, making it impossible. There's a reason tow trucks hook up vehicles by the drive axle ends. (00:08:40)
Stupidity: Colette was holding the metal ladle close to the bottom of its long handle and near the top of the hot pot of stew she was serving, and yelled, "Ah." due to her hand getting too hot. She had a towel wrapped around the handle of the pot, but it was that hand that got too hot/burned (not the hand that was holding the metal ladle close to the top of the steaming hot pot of stew). (00:04:48)
Stupidity: While awaiting their flight home, Nancy told Teri, "Think I miscalculated the tea. I'm not gonna last three-and-a-half hours [on the non-stop flight with no restroom facility]." Nancy then went into the gift shop where Stuart Couch introduced himself, asked if he could get her a drink, and bought her a bottle of beer. Nancy drank at least some of the beer on screen, even though she was already concerned about drinking tea before her flight. (00:59:28 - 01:00:30)
Stupidity: Instead of breaking into Gordo's house, covering all the furniture in one room with sheets/thin blankets, and waiting for Gordo to return home in order to kill him via a "dramatic re-enactment" (in an adjacent room which was not shown to the audience), Bill could have taken Gordo anywhere to avoid getting caught or calling the police on himself. Had Bill taken Gordo to his already-"wrapped" motel room, he had enough injuries after the fight to claim self-defense against an intruder. (01:43:20)
Stupidity: Patrizia Reggiani is no rocket scientist in this movie, but it's quite bizarre that calling the fortune-teller she sees on TV, with her name written in giant letters in two different spots on screen, she asks her "What's your name?" It makes sense for the international audience who may not recognize the word "Pina" as a name, but an Italian wouldn't have any doubt. (00:41:05)
Stupidity: Lea had two "Notice of Auction" printouts (the correct one and the one she lied about to detectives) in her small purse when she had Casi meet her at a restaurant. Lea excused herself to go to the bathroom but did not take her purse with her. Women almost always take their purses with them and Lea should have, considering she had papers inside she didn't want Casi to see. While Lea was gone, Casi looked at the printouts and then knew Lea was lying to him about the Navigator's auction date. (00:32:33 - 00:35:04)
Stupidity: Lyla does not like the idea of Mickey fighting and initially does not want him to agree to the fight. Lyla eventually tells him "okay", but he must stop when she tells him to stop. This may give her a reason to go to Dubai, UAE with him, but she is about 8 months pregnant. An international flight has some risks plus she would be far away from her OB-GYN doctor if there were a complication. She should have had enough sense to avoid international flights/traveling so close to her due date.
Stupidity: Two things: 1. I find it inconceivable that there is only one Carbon Dioxide Removal Assembly (CDMA) and no spares for a two-year mission. 2. The crew has 20 days to fix an oxygen problem. The commander decides on a 10-day window. 5 days into that window they try to load oxygen from the Kingfisher spacecraft. One is lost into space. Zoe decides to go and fill the other one in the midst of a solar storm that will only last hours. Why take that chance? A few hours wait is still quite possible.
Stupidity: During the lightning storm, Hannah and Connor did not stop running through the open field when they reached the low point (dip). Hannah kept telling Connor to run, stop, and crouch even when they were headed uphill. It would have been safer to crouch at the lowest point in the field. Connor fell a couple times and Hannah also fell (or got struck) - both ending up doing one of the worst things - lying down on the ground instead of minimizing contact. (00:53:39)
Stupidity: For claiming to know how to deal with Music Ebo doesn't do a good job. He wrestles her to the ground to calm her down which is very dangerous for both the patient and the caregiver. It's only recommended as a very last resort and in both scenes with this neither one required it to be used.