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)
Stupidity: The marble somehow got under Don's skin at the bottom of his leg and was moving up the length of his body. Instead of heading off the marble in order to intercept and stop it from continuing up his leg, Don got a knife/box cutter and jabbed it into his skin near the bottom of his leg and trailed the marble, causing a long gash that served no purpose (other than creating a lot of blood and making a gory scene). The marble eventually exited his body via his eye. (01:04:40)
Stupidity: If there was a live camera feed right where the eggs were, one wonders how everyone missed the 'small' fact that they hatched. That of course combined with the fact that going for no reason in the nest of the creatures that nearly killed another crewmember is suicidal, and Bob goes with no weapons and with one small plastic fruit crate to fetch eggs that are bigger than an ostrich's. (00:55:25)
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: Becky possessed the mysterious key that the four neo-Nazi prison escapees were at the lake house to retrieve. Becky didn't know what the key was for, so did not have a vested interest in it. By first denying that she knew where the key was and later refusing to give it to Dominick, her father was killed. Had Becky simply handed over the key when Dominick asked for it, it is possible no-one would have been hurt/ killed. Becky keeping the key with no importance to her served to further the plot.
Stupidity: In the opening scene, Sonny has come to her now-deceased father's house. Police and the coroner are in the basement taking notes while standing around the body - which is face down and awkwardly lying at the bottom of the steps. Det. Sabatino informed Sonny that the death was ruled accidental due to a high blood-alcohol level. Detectives or the coroner did not rule out other possibilities: natural cause, suicide, or murder. High blood-alcohol level could be secondary.
Stupidity: On the phone, Marion told Paul, "I'm nearly certain she'll [Maggie] get full custody, but if you fight this, you stand to lose everything, even visitation to see your daughter. Sign the contract..." Jeff wanted to see Jenny and, other than being unemployed for three years, there were no reasons his parental rights should be terminated. It was not "in the best interests" of Jenny to relocate to Seattle and never see her father again. Fighting it, Jeff would have likely won custody of Jenny. (00:15:39)
Stupidity: Possessed child Nicholas made the cross from a necklace pierce Fr Louis in the neck, causing blood to gush out. Fr Peter yelled, "Fr Louis, Fr Louis" as Fr Louis tried to pull out the cross. No one else present rushed over to try to help stop the bleeding or assist him. Fr Louis' sash could have been used around his neck to apply direct pressure to the wound. (Although it might have been obvious that it was a fatal wound to the neck, someone should have at least tried to help). (00:07:15)
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: 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: 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: 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: 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)