
Stupidity: Five friends, apparently in their mid-20s, went to a remote log cabin near a ski resort to celebrate the New Year, but none of them thought enough in advance to bring an emergency supply of food (or even snacks for their return trip), flashlights/batteries to use in the event of a power outage, or make sure they had ample gasoline for the generator.

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: 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: After Caleb escapes the vampire "family" with his father and sister, he undergoes a blood transfusion which cures his vampirism. As soon as this was done Caleb, his father and sister should have fled somewhere else immediately. The vampire "family" knew where he lived as they kidnapped him in front of his home. Instead, they stick around the home, with no obvious intention of leaving. Of course, the vampires track them down and kidnap Sarah.

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: 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: Throughout the movie, and especially up on the top of the apartment building before they get rescued at the beginning of the movie, Gerry counts 12 seconds to check if someone has been infected. Just to mention this apartment example, he gets blood on his face and spends more than 12 seconds getting up and to the edge of the roof, plenty of time to become "Zeke." He then counts 12 seconds, as if they would actually matter to his infection.

Stupidity: When Joey is trying to escape from the Cenobites, she bumps into a man who calls her "Baby" and wonders where she's off to in such a hurry, possibly because he's looking for a booty call. The streets are literally on fire (there's a huge flame right next to the man) and buildings are exploding. Even if he's extremely horny, he should still be able to tell a woman doesn't want to stay around with so much chaos and destruction happening around them.
Suggested correction: The dude was probably high on various types of drugs and probably didn't even know where he was. Just a thought.
Too high to recognize buildings exploding and fires bursting from out of nowhere, but not too high to recognize a woman in distress?
He didn't recognize much did he? Well, he saw she was female. But not really what was going on.
He recognized she was off somewhere in a hurry. He also had a pretty instant reaction to seeing the Camerahead Cenobite.

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: 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)

Stupidity: The murder during the opening scene is ridiculous. Angela chases a screaming woman down an alleyway, in broad daylight, in front of crowds of witnesses, while driving a truck, runs her over, then throws her body in the back of the truck and then waits right next to the alley where she murdered the girl to be picked up by the van headed to the camp. At no point did any one of the numerous people on the street try to help the girl or even just investigate the alley to see if she was hurt (supposing it was an accident) or just call the police?

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: 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)