Stupidity: The protagonist with her kid stops a car with two guys, in the middle of the night in an isolated country road. Both the adult and the kid scream that a dangerous maniac is chasing them. So naturally, the two guys (who do believe them, it's not that they are being dismissive) offer them some hot coffee, and they drink it right there in the middle of the street, not dialling for help, getting in the car to drive away or anything, because that's what normal people do when you tell them a serial killer is nearby; stop for a hot cuppa.
Stupidity: Soon after the new art therapy instructor Anna arrived for her first day at the mental hospital, Leena - the most dangerous patient - was not in her room and nobody could find her. The hospital went into lock down and the doctor took Anna to a room. Without first looking in the room, the doctor said, "Do not leave this room. Don't worry, you will be safe here and we'll come back after we found her" and then locked Anna inside the room - with Leena. (00:03:36 - 00:04:41)
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: The cleaning crew can't open doors and stand in front of a large window. They see a couple walking a dog, so they scream, "Hey!" and wave their hands in the air, even though the people are not looking their way. They do not bang on the glass or use their flashlights to try to draw attention. (00:29:13)
Stupidity: Judy Hicks, a trained cop and sheriff with over a decade of experience, does not carry a back-up holstered gun on her when she is off-duty, one day after her son's close friend is stabbed seven times near to death. A trained police officer/sheriff would also question why the killer would tell her they are about to kill her son, before they do it, if not to bait them and lure them into a trap. (00:45:58 - 00:46:30)
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: 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: After Danny kills Kat on the sidewalk, he just stands there with Rory while the other gang members flee to their vehicles. William comes running out of the Mexican restaurant and Rory tells Danny, "We gotta go, brother. Now." Rory sees William and gets into the vehicle without getting Danny to go with him. The gang drives away with Rory watching out the window as William tackles and fights with Danny. Rory and the rest of the violent gang left behind someone who just passed the initiation. (00:07:10 - 00:07:40)
Stupidity: At the police station, a bucket of tools was left along the wall within a couple inches of the jail cell door where so-called "Amityville Strangler" Joe Gallo was detained. Gallo was able to reach the tools, use something to unlock his cell, and have a weapon - sledgehammer - "to go." (00:46:57 - 00:57:30)
Stupidity: David was seriously injured by Olivia a few times (e.g, hit by her car and on the head with a baseball bat). When Olivia went into the bedroom, David was able to knock over his chair, get a knife off the floor, and free himself from being duct-taped to the chair. David took the car keys but left the knife on the floor, even though he was weak and vulnerable and Olivia had previously chased and caught him. When David returned to the cabin, Olivia came in with the knife and keys. (01:02:55 - 01:09:49)