Stupidity: Nobody is ever going to take a nine-year-old child on a dangerous mission to locate and destroy a marauding, giant shark, a project that has already cost a number of lives.
Stupidity: When Greg volunteers to watch the security monitors to see if the creatures can affect people through a screen, no-one even thinks to remain in the room with him to cover up the monitors with a blanket, towel or something similar in case the creatures' power can be transmitted that way. They tied him to a chair to reduce his chances of committing suicide, so they certainly thought there was a possibility that he would be in danger.
Stupidity: Zoe and her team go into the medical university to get medical supplies for their camp and Lily. As they are leaving, Zoe sees the Epidemiology room where she used to work and waits, letting her team run off leaving her behind and then goes into the room alone so she can get pictures and some small bottles. Letting her team run off leaving her by herself in a zombie infested building in a city without telling her team that she's not following them. And this did lead to her having to shoot at a zombie that jumped her, alerting the other zombies around, and is what got Frank killed and lead to the smart zombie stowing away on their Hummers and sneaking into their camp. (00:29:35)
Stupidity: During the ghost chase montage, why would Velma look for her glasses with her hand in midair instead than on the ground? No matter how nearsighted you are, the closest wall, or obstacle, is meters away and you'll never find your glasses anywhere but the floor. She does it for a longer time than anyone would. (00:52:40)
Stupidity: When Olivia said, "Momma. It's hot", she was wearing a long-sleeved shirt over her overalls and shirt. Her mom, wearing a sweater over a tunic top, replied, "I know, baby" and asked Steve to turn the A/C on in the RV, but the A/C didn't work. Instead of removing some layers, Steve opened some windows. Jay complained how hot it was, but he wore a hooded sleeveless sweatshirt over his T-shirt (and never took it off, even when outside in the desert). (00:09:31 - 00:46:00)
Stupidity: Milton, straddling Edie's body on the floor, is about to pull the plastic sheeting over her head. Edie's eyes suddenly open, and she starts screaming. Instead of quickly pulling the plastic sheeting over her head and suffocating her, Milton half-heartedly struggles to control Edie's hands, which she somehow manages to raise from alongside her body and around his to be in front of her neck. (00:38:50)