Stupidity: Daphne and Velma's adventure in the corridor behind the school lockers begins when they enter the theater backstage from a random door opened by a button that is lit up in the dark, effectively negating the "secret tunnel system" status. Why would anyone build a corridor with an entrance into school lockers so secret that it's invisible from the outside, and then put a button-activated entrance to it in another location, even making the button a fully lit one shining in the dark?
Stupidity: How would a devout catholic like Angela not believe in the devil, as she tells Constantine? Makes no sense.
Suggested correction: Some Christians don't believe in the literal personification of the Devil. They see him more as a metaphor for all human sins. So according to them, the Devil is not something to blame evil on. Everybody has God and the Devil in them.
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: Quinn wasn't able to kill Dr. Sullivan, so decided to kill herself to save her sister (by dying earlier and ending the curse). Instead of telling her younger (teenaged) sister about the Narcan and how to use it, she writes Narcan on her upper arm, assuming her sister will see it in time, know that there is a syringe in her pocket, and not hesitate to inject the Narcan. Quinn left too many steps up to chance and did not take the necessary precautions to help insure she could be revived. At the same time, a registered nurse - even a recent graduate - should know that this would not be the circumstances under which Narcan might be successful in counteracting the morphine. Taking time to say, "I love you, Jordan. I'll always be with you" was wasted time that should have been used giving her sister instructions because Quinn (erroneously) believed her plan would work. (01:17:08)
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: 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: 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: When Bobbi Jo sees Ash's severed possessed hand holding her hand, she freaks out and runs out of the cabin and then gets killed by possessed trees. While it's certainly natural for her to be terrified, why would she run AWAY from where all her friends are and go off by herself?
Suggested correction: She is a civilian in a blind panic, and even people with actual training have done dumber in panics (e.g., choking firefighters run away from other firefighters who could help them). She also has no idea how dangerous the woods are at this point; she walked to the cabin just fine and only saw scary things inside it, so she thinks she'll be fine if she just runs from the cabin.