Stupidity: If the song "Beyond the Sea" can be continuously broadcast on the radio station as a "cryptic clue" that there are survivors living on the island and the aliens are unable to swim, then someone could verbally announce that information over the radio and otherwise communicate with other humans. The same thing about how Regan's hearing aid incapacitates the creatures. Rather than explaining how to weaponized the high-frequency sound, only the hearing aid's noise is broadcast and anyone hearing it has to figure it out.
Stupidity: The guy who offered to help Rachel go to her car at the gas station and get away from road-raging Tom walked directly in front of the path Tom would be taking to leave. When the guy realised Tom was going to run him over, he went onto the road instead of toward the store where he would be safer. Also, the guy - knowing Rachel's situation - foolishly told Tom that he got his plate number, which served to aggravate Tom into driving into him. (00:32:15)
Stupidity: *SPOILER* Cecilia does not want James (or anyone else) to know she uses/wears the "invisibility suit", which, recorded, makes it look like Adrian slits his own throat. Cecilia leaves the invisibility suit in plain view near the top of her open purse/bag, where James notices it and knows she kills Adrian. (01:51:35)
Stupidity: Crystal knows she is being hunted and in a very dangerous life-threatening situation yet, after she kills "Ma and Pops" in their Main Street Market, she grabs only a few shotgun shells out of the open box instead of taking all of them or as many as she could find. Also, she walks some distance to the store but did not take anything to drink or eat. (00:27:00 - 00:27:55)
Stupidity: Molly was being hunted and didn't know if she could trust Rayburn when he encountered her. Rayburn told Molly that he wanted to help, wouldn't hurt her, put a blanket around her, and even carried her in the woods for some distance. When he put her on the ground (along with his backpack and rifle) to go find the rope to open the pitfall doors and had his back turned, Molly got out from under the blanket and hid nearby. Rayburn quickly found Molly, and Molly made a lame attempt at attacking him with some kind of club. The next day when Rayburn and Molly were in Rayburn's house and her hunter showed up and attacked Rayburn, Molly grabbed Rayburn's rifle and fired several shots at the hunter. Molly obviously knew how to use a rifle, so it was asinine for her to have hidden from Rayburn the previously night without taking his rifle (or the blanket). (00:36:22)
Stupidity: Throughout the entire movie the police just don't seem to be doing their job. From the beginning they never taped off the RV as a possible crime scene, for 4 days every night the police always have their cherries on, and they are standing in front if the lake looking in the same spot about 50 yards away from the RV. They never searched the buildings on the grounds; which the mother searched and found the sicko responsible for making filthy movies. They also never searched the trash. Doesn't jive. (00:00:01 - 01:50:55)
Stupidity: A zombie is climbing up the apartment wall and is almost at Aidan's balcony railing. Instead of trying to push the Zombie off while he had the advantage (and there was a surfboard on the balcony that could have been used), Aidan ran inside and locked the door to the balcony. This furthered the plot and added excitement by having zombies coming at Aidan from multiple directions. (00:43:09)
Stupidity: Early in the movie, AMMO and its crew show remarkable prudence in their approach to the drug deal, and a point is made about them following procedure, planning accurately using technology and trying to minimize collateral damage. The sting at the club goes against all of that. At the end of the day, it just consisted of getting close to the target and pulling guns on him in the middle of a crowd of civilians, the exits of the place (the front door even) completely unguarded.
Stupidity: This lady lives in the United States, she obviously doesn't have a criminal record, she can go buy a handgun or rifle, heck she can buy a handgun and rifle and extra magazines and ammo to defend herself, but she chooses golf clubs and baseball bats and mace. (00:00:01 - 01:20:00)
Stupidity: Cole fell into the lake, and he said he couldn't swim. Instead of making any move to avoid being mangled by the boat motor Becky was aiming toward him, he just stayed in the same spot and was killed. Even if one cannot swim, it would be possible to hold one's breath under water for a while to escape the motor's range. It also seemed like he was close enough to the dock to simply lunge forward and grab onto it to stay afloat. Cole was close to the lake's edge - the water shouldn't be deep.
Stupidity: Matt told Joey they could not check on the guard in the hole because they had committed the "perfect crime" but could get caught if the guard got out. Matt later told his friend Tom "everything" about stealing from Ms. Dillard. Bragging about what they did is certainly no way to help ensure they would not get caught.
Stupidity: Sam slid under the truck to narrowly escape the lioness. Even though there was more than enough ground clearance, the lioness did not go under the truck to attack Sam, as though the truck's frame were a barrier. The lioness could have easily crawled under the truck (unless a CGI lioness is not as spry or nimble as a "real" lioness). Sam being under the truck would not constitute adequate protection from an attacking lioness. (01:10:30 - 01:35:20)
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.
Stupidity: When Liz sees the dead deer blocking her lane, instead of merely going around it (using the opposing lane on a rural road without jeopardizing her safety), she exits her vehicle - leaving her three-year-old daughter unattended in the back seat - to "look" at the deer. This furthers the plot by giving the "werewolf" opportunity to attack and kill her... as well as her daughter Miley. (00:42:35)