Stupidity: Charlie set her mom's blouse sleeves on fire and mom/Vicky just stood in the same spot, holding her arms (burning sleeves) out in front of her until Andy came and smothered the flames with a blanket. Even children know to "drop and roll" or put fire out by smothering it. (00:22:56)
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: Dr. Emma Collins and Shaw were in the water with Sally the Great White Shark. Emma put her right hand over Sally's nose and pushed her back. Sally swam forward and Emma pointed her index finger at Sally's nose then pushed her back again. Emma told Shaw, "If you stay calm and redirect" and "Don't try this at home" before Sally pushed Emma with her nose on Emma's stomach and rammed her into an underwater object. Emma was the instigator, goading a Great White Shark. Emma could have been killed. (00:05:10)
Stupidity: After leaving the movie theatre, Billy and Jenny have already reached their parked car, opened the car door, and are about to get in when they are confronted by her ex-boyfriend. The next moment she says, "Just walk away," and she and Billy do just that, ostensibly to ignore and avoid the ex-boyfriend. Why didn't they just finish getting into the car and drive off? Where are they walking? It makes no sense.
Stupidity: Since we get frequent POV shots from their eyes, we see exactly what the creatures hunting Venom see. Even when the Codex disappears from their view, they can see distinctly Venom's shape, and they have no problem using their appendages and brute force against any and every creature. There are moments in the movie (such as the scene in Vegas) when the Xenophage just stops for no reason; even if the big blinking thing is gone, you can still see the shape it was attached to.
Stupidity: When one of the robots gets stuck in the cave, the group moves past it and doesn't bother to try to disable it or take its weapon. A trained Navy SEAL would never leave an enemy combatant just lying there with its weapon within reach.