Stupidity: When Amy heard Luce coming toward the kitchen, she "hid" his school paper and bag of fireworks in a kitchen cupboard, apparently while knowing Luce would be doing the dishes (and putting them away in the cupboards). Amy and Peter were on the patio (with the door open) just outside the kitchen when Luce was doing the dishes yet Amy made no attempt to prevent Luce from finding his paper and bag of fireworks (which he did). (00:16:20 - 00:21:00)
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: Dirk squeezed a long stream of lighter fluid onto the pile of Mario's clothes, then saw a necklace, which he removed (and potentially got lighter fluid on his hand while doing so). Instead of tossing a match from a safe (r) distance, Dirk crouched next to the saturated clothes and held a match near the bottom of the pile, risking burning himself. (00:06:40 - 00:07:21)
Stupidity: Colette was holding the metal ladle close to the bottom of its long handle and near the top of the hot pot of stew she was serving, and yelled, "Ah." due to her hand getting too hot. She had a towel wrapped around the handle of the pot, but it was that hand that got too hot/burned (not the hand that was holding the metal ladle close to the top of the steaming hot pot of stew). (00:04:48)
Stupidity: Gunner [Director and Co-Writer Brock Harris] was given a puppy ("Barnaby") by the nurse (?) who told him he was sorry his "[VA] benefits didn't go through." Gunner could have merely refused to take the puppy but, instead, drove away with it - and later threw it out his truck's window while he was driving. (00:24:42 - 00:25:55)
Stupidity: One of the major reveals and plot twists of the movie lies in the fact that a certain character is an accomplice of the medium. The reveal, though, does not explain at all how said individual managed to fool Poirot; the trick briefly shown involves a simple rope pull to open a door. Poirot was looking exactly for that kind of trick, paces around the room constantly on maximum alert, and examines the door itself later. Likewise, it does not explain where and how they could hide the stuffed animal.
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: Kora and Gunnar arrive at the town to find the rebel Bloodaxes via a contact of Gunnar's. They literally see this contact being hauled out of a bar by bounty hunters who work for the Imperium...and then walk straight into the same bar and start openly talking about needing to find the rebels right in front of the bartender and other patrons, who for all they know are all Imperium-aligned.
Stupidity: During the battle on the harvester, Chani is firing an anti-aircraft projectile at a Sardaukar approaching her. Although the projectile is strong enough to destroy an ornithopter, instead of blasting the Sardaukar away, it pushes it into the air until it hits the harvester track. (00:36:39)
Stupidity: When Eitan and Ran escape the Egyptian troops, they take shelter in a creek and fire back at them. Ran is firing from his handgun, but the troops are at least a couple of hundred meters away, while a common handgun can only reach a range of about 30 meters. There is no way the handgun's bullets will even reach the chasing troops. (00:57:24)
Stupidity: When learning how to gut and skin the buffalo, Will gets sick but goes to fill up a bucket of water from the river. He gags and partially throws up, spitting into the river... and then immediately scoops up water in the bucket in the very spot he just threw up into. (00:48:20)