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: Soon after the new art therapy instructor Anna arrived for her first day at the mental hospital, Leena - the most dangerous patient - was not in her room and nobody could find her. The hospital went into lock down and the doctor took Anna to a room. Without first looking in the room, the doctor said, "Do not leave this room. Don't worry, you will be safe here and we'll come back after we found her" and then locked Anna inside the room - with Leena. (00:03:36 - 00:04:41)
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: When the plane begins crashing, Kayla tells Claire that she is sitting in an ejector seat. She also mentions that there are no parachutes because she "wasn't expecting company." If Kayla was expecting to be alone in a crash scenario, what use would installing the ejector seat in one of the passenger seats have been? She would have to move from the pilot seat to the passenger seat while the plan is crashing. Surely it would've been easier to install the ejector seat in the pilot seat.
Stupidity: At the police station, a bucket of tools was left along the wall within a couple inches of the jail cell door where so-called "Amityville Strangler" Joe Gallo was detained. Gallo was able to reach the tools, use something to unlock his cell, and have a weapon - sledgehammer - "to go." (00:46:57 - 00:57:30)
Stupidity: After the cloth was lit, the "Santa" could have easily thrown the Molotov Cocktail far enough to ignite the Christmas tree but, instead, casually walked to get closer (within several feet) before tossing it. This may have been for dramatic purposes, but carrying a lit rag (part of a Molotov Cocktail) unnecessarily increases the risk of getting a burn injury (which can be quite serious/painful). (00:02:35)
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: 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: 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: 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)