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: 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: Once Owen and Kayla have left the crashed plane, Kayla hands Owen a tracking device. She reveals that the ejector seat was installed with a beacon, and that the device is used to track the beacon. However, Kayla mentioned earlier that she only ever intended to use the ejector seat for herself, so there was no real reason for her to have installed the ejector seat with a beacon, unless she planned on tracking herself.
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: 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: 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: The protagonist is wearing normal boots atop a spacesuit while being on a moon mission. (01:37:11)