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: After Danny kills Kat on the sidewalk, he just stands there with Rory while the other gang members flee to their vehicles. William comes running out of the Mexican restaurant and Rory tells Danny, "We gotta go, brother. Now." Rory sees William and gets into the vehicle without getting Danny to go with him. The gang drives away with Rory watching out the window as William tackles and fights with Danny. Rory and the rest of the violent gang left behind someone who just passed the initiation. (00:07:10 - 00:07:40)
Stupidity: There is a map of the area on the wall behind Jason as he tells Sam and Ben to "look for anything useful - maps, food, radios, anything." Also, there is a map in the drawer as Sam opens it, but she removes the notice letter "shuttered for budget cuts" and the snowmobile keys. (00:44:43 - 00:45:20)
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 protagonist is wearing normal boots atop a spacesuit while being on a moon mission. (01:37:11)
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.
Stupidity: Grace (?) came out of hiding holding a fire extinguisher upside-down as though prepared to use the fire extinguisher to whack any Allosaurus that approached her. Actually spraying the dinosaur's eyes with the chemical contents of the fire extinguisher - and from a greater distance by using the hose - would have been much more practical and effective as a repellent and thereby less likely to result in getting maimed/killed. (She did not use the fire extinguisher at all). (01:02:20)
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: Pettis and his right-hand man Spade are in a room watching Candace, who is tied to a chair. Immediately after sending Spade to get food, Pettis tells Candace he is going to "relieve himself", thereby leaving Candace alone and giving her the opportunity to escape (which she does). (00:59:50)
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)