Stupidity: If the ninja can inhabit a dead body (and one that has been dead for a LONG time, even) without any penalty in his agility and powers and it is stated that only another ninja can truly kill him (which surely seems correct, since he would still fight even when hit by hundreds of bullets), the whole movie plot that involves him possessing Christie - who is a reluctant and part-time host - does not quite make sense.
Stupidity: Billy and Allie saw there were several men in the area also searching for the medallion and the kids even hid from the men minutes before finding the medallion. Instead of quickly sneaking away after they found the medallion, Billy and Allie stayed in the same spot and Billy shouted, "We found it! We found it!" Seconds later, two men came from behind the tree. (00:24:00 - 00:24:21)
Stupidity: The protagonist is wearing normal boots atop a spacesuit while being on a moon mission. (01:37:11)
Stupidity: The "keep everyone in the stadium" plan is foiled when gold is scattered around the outside and everyone leaves. The bad guys didn't consider locking the doors? Not least because once the terrifying sky tendrils descend and boiling red clouds start filling the arena, surely people would have started bolting for the wide open exits.
Stupidity: Given Enchantress' power, why not just try to cure Dr June Moone, so she can't threaten the world? That's what the government should've done.
Suggested correction: We're talking about Amanda Waller who had no issues murdering innocent government employees that were accidentally exposed to her top-secret Suicide Squad program they weren't cleared for (versus making them sign an NDA not to talk about it as in a real-life accidental exposure to uncleared employees). Waller just took the easier option which was to threaten to kill Enchantress if she didn't obey versus spending countless dollars (and exposing more people to Waller's program) trying to cure her.
Stupidity: I won't begrudge Father Lamont for instinctively grabbing for the first thing in reach to try to smother the box on fire (even if it is a pair of wooden crutches), but you'd think at some point he'd notice that he's just spreading the fire and making it much, much worse, but he keeps at it until the doctor finally returns with the extinguisher.
Stupidity: Right before the movie's final battle, Sub-Zero taunts Cole with his wife's necklace and makes him chase him into a portal. Sonya was right next to Cole, and she did not come along with him. They had previously planned to take on Sub-Zero together, but apparently she just left him to chase Sub-Zero alone to an unknown location.
Stupidity: If the weak spot of the house is the fiery chimney, then the kids could have just thrown the dynamite into it instead of lighting it themselves. They put themselves in unnecessary danger for the sake of drama.
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 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.
Stupidity: When Governor Swann is trying to encourage his daughter while she's locked within the ship's chambers trying to escape, she climbs down to a boat. Nobody in the Navy would have left a boat there, neither would Jack (and why would he, especially without anyone noticing?). The boat just mysteriously appears to help her escape.
Stupidity: Even though Hermione reminds them that they must not be seen after freeing Buckbeak, they fly toward the castle, whooping and cheering because they're flying. Then, they land in the courtyard of the castle, have a conversation, and then Sirius flies off with Buckbeak. Not the behavior of trying to remain unseen and unheard.
Stupidity: Jake uses several Loops and travels all over the world in order to get back on the boat at the end of the film. This was completely unnecessary as only the final Loop in 1942, which was available to him in 2016, was the one he needed to enter despite the fact that its nature should have prevented him from reaching 1943 anyway.