Stupidity: The humans return to Pandora knowing how things went last time...and still haven't improved the cockpit glass to be able to resist a bow and arrow?
Stupidity: The whole premise of the plan is summarized by Helen in the sentence "They don't know Andi is dead, so why would they suspect anything?" However, "they" conspired in unison to financially ruin her sister, and "they" did not answer the mail, none of them. The possibility that "they" all could have gathered and silenced her by force is one of the most likely ones, and yet neither she or Blanc consider even for a moment that the literal crime conspirators could have conspired.
Stupidity: Gargantos neutralizes Strange's cape tossing a motorbike at it. Hilariously enough, someone was riding the motorbike and Gargantos knocked him off the bike. The streets were littered with all sorts of wreckage; the biker who made a conscious effort to drive around all sorts of obstacles literally blocking 90% of the roads and ride right at the enormous monster in plain sight must have been in a hell of a rush.
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: Alex, a contract killer, went to the hospital to kill a patient. After parking in the hospital garage, Alex put his car keys under the sun visor, risking the possibility that his car could be stolen. When Alex returned to his vehicle, the keys were not there. A professional or expert assassin should take precautions to assure his getaway car will be there and keep his keys on him. (00:01:45 - 00:04:30)
Stupidity: How did Elvis not know that Gary wasn't in his house? Gary said he just put 2 bullets in Elvis' poster. And why wouldn't his wife tell Elvis they weren't at home? And what happened to the cops who busted into his home and found nobody there? Why wouldn't they go to the station?
Stupidity: Simi sets the wooden tepee structure on fire with Claudia next to it, and Claudia's clothes get engulfed in flames. Claudia continues to stand next to the fire, flailing her arms instead of following the commonly-known "learn not to burn" procedure that is taught to children: "Stop, drop, and roll" to extinguish the fire on one's clothes - and to stay away from fire. (01:31:00)
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: David was seriously injured by Olivia a few times (e.g, hit by her car and on the head with a baseball bat). When Olivia went into the bedroom, David was able to knock over his chair, get a knife off the floor, and free himself from being duct-taped to the chair. David took the car keys but left the knife on the floor, even though he was weak and vulnerable and Olivia had previously chased and caught him. When David returned to the cabin, Olivia came in with the knife and keys. (01:02:55 - 01:09:49)
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: Mia said, "Look, there's a clinic. Are they open?" The Thief replied, "I don't know, but we gotta get off the road" (because Temple and his goons were chasing them and not far behind). The thief parked in front of the clinic, which was along the road, and he and Mia went into the clinic. (00:39:38)
Stupidity: While Reed and Jackson slid the fiberscope (or borescope camera) under the door and the whole time they were looking into the room, they stood in front of the door instead of moving to the side. If the perpetrator saw the camera and shot through the door, Reed and/or Jackson could have been seriously injured or killed. (01:03:05 - 01:04:43)
Stupidity: Dr. April (as well as her patient Mrs. B) is nonchalant when her young nephew Zeke brings his dog hit by an automobile into the hospital. Dr. April, continuing to hold her clipboard, tells her nephew she is "not that kind of doctor" and "what you need is a doctor for animals." The doctor-aunt could have at least taken the dog from Zeke's arms to ease his panic and see if there is something obvious she could do to help the dog. Instead, she walks out with Zeke still carrying the dog. (00:02:45)
Stupidity: After eating the bird, why didn't Becky send her phone down inside the bird, but instead climbed down to put her phone inside Hunter's body?
Stupidity: As soon as Bundy got into the building (instead of when he was still outside), Kaminsky asked him for a "camera check." Bundy had a yellow button with a big star in the center pinned to his jacket's right lapel. Bundy put some fingers next to the button and raised his lapel with the others, making it obvious he was wearing a camera. When Manson got into the lobby (area likely to be under video surveillance), Reese (who saw Bundy and Manson outside) came into the building to give them weapons. (00:03:50)
Stupidity: After the Prince and his entourage unexpectedly arrived at the chalet, Ms. Tilson told Emily that she tried to call but Emily's phone was turned off. Emily replied, "I forgot to charge it last night." A woman (or person) who went to a remote vacant chalet in the mountains during the winter, contracted to watch over the place, would not "forget" to charge her cell phone. Making sure her cell phone was charged (and has reception) would be at or near the top of her list of concerns. (00:09:10)
Stupidity: When Poirot's team attacks the Germans, they do it through the cover of gas they release. The Germans are having a good time chatting and not taking the situation seriously when there's a big cloud of noxious gas advancing towards them. Despite Poirot's oh-so-clever plan, they should by all means know that something is coming their way, but they get caught entirely with their pants down.