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: 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: 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: Don told Allen he'd have to wear a blindfold to his house because he didn't want Allen to know where he lived. Instead of having Allen ride in the cab where he could monitor him to make sure he didn't remove the blindfold, Don made Allen ride in the truck's bed (where Allen did lower the blindfold to use his cell phone). (00:14:24 - 00:17:00)
Stupidity: The cleaning crew can't open doors and stand in front of a large window. They see a couple walking a dog, so they scream, "Hey!" and wave their hands in the air, even though the people are not looking their way. They do not bang on the glass or use their flashlights to try to draw attention. (00:29:13)
Stupidity: Despite being wanted for murder, escaping jail, and having his face plastered on the front page of the newspaper, Morbius meets Martine in a public diner and doesn't even make an attempt to conceal his identity while they are seated. Even shaving off his beard and cutting his hair short would have gone a long way in making him more unrecognizable.
Stupidity: When Joaquin was running from Cornell, he turned into an alley between buildings. He was to the left (of the screen), which was unobstructed, but ran to the right, so he could jump over boxes. Cornell could have also stayed to the left, but ran far enough to the right where he could kick the boxes out of his way. Easily avoiding the boxes would have been faster. (00:11:37)
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: 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: Catherine, wearing a dress and coat, was outside in the snow chopping firewood while her teenaged son sat nearby playing video games on his tablet. This could also fall under "plot hole" because having the son close by made him available to try to help his mom when Pryce attacked her (with the intent to rape). (00:36:40 - 00:37:15)
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: 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: 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.