Stupidity: Mason should have also removed the other gun he had in his office for display, or he could have removed the bullets from the guns. There is no reason he would have to use these guns for his victims, so it makes no sense to leave the other gun in his office or have them loaded with bullets.
Stupidity: When Chili, the hippy stoner chick, goes upstairs and sees Jason holding a fire poker, she runs back downstairs and heads to the door. Then the door gets blown open by the wind...and then she runs back inside for no particular reason and gets killed by Jason.
Stupidity: When Drake cuts off power to Hayes' apartment, the cops can't physically make a landlord abide by the laws, and seeing a judge would take months. In the meantime, by keeping the power off in the apartment, Hayes would have had no choice but to abandon his scheme and move on to another victim. With no power in the apartment, it's unusable and unlivable.
Stupidity: Wes is characterized as one of the more careful characters early on, buying both a stun gun and pepper spray. When he comes home after the meeting where the dangers of Ghostface were further laid out, he proceeds to set both his weapons down and takes a shower after his mom tells him he'll be alone in the house, which is exactly how Tara and countless victims of previous sprees were attacked. The fact he even lived through his shower is lucky enough.
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)
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: Nicholas Cage saves the kids coming out of the dark dimension exactly at midnight but gets back home at dawn. It should be noted that he has two other kids to bring back home and if we assume he did that already, it would explain part of the time discrepancy. In that case he kept his wife up all night when she was fearing for the safety of both without a word, but she's not upset or anything.
Stupidity: Harrigan leaves his partner's badge on his grave where anyone could take it and use it to impersonate a cop or abuse it in other ways. (00:50:15)
Stupidity: The location of the small hole near the bottom of the wood-trimmed wall where Everette passed Sharyn a note and pencil would have taken much more time and effort to accomplish compared to merely penetrating the wall a little higher (where there would not have been wood trimming/baseboard on both sides). (00:10:00)
Stupidity: The chef/cook of the house has his rifle, and he knows Morgan lost control and pretty much killed everybody on the premises. He comes face to face with her, but he lowers his rifle instead of taking the subject (Morgan) out with a shot. There is no logic here but to only pro-long the film. There's no reasoning with a scientifically created subject which is killing everybody it comes into contact with. (01:09:00)
Stupidity: When Joan is in hell and is confronted by the overlord, she attempts to shoot him with the BFG-9000, so the overlord uses his powers to telekinetically blast Joan away and disarm her. A minute later, Joan picks up the BFG-9000 again, but the overlord never considers using its powers to blast Joan away from him again, which results in him getting killed.
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: 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: All those people at the bus station yet no-one reports a blood trail from the phone to the bathroom? There's even blood in the sink.