Stupidity: To save the people from rubble falling on their heads Flash does not move them aside like he did with his future love, but does some strange manoeuvre which takes seconds - since we see people moving normally - and happens in midair (how does he get there?) and that does not take care of a huge chunk of concrete which he does nothing about, shouting just "No!", but Cyborg has all the time he needs to load his Mega Man arm and vaporize it in midair. (02:00:00)
Stupidity: When the rebel leader talks to Rick Flag guaranteeing her help, she mentions that she will help to find "this Gaius Grieves", speaking as if it was someone unknown to her. He's been around the island (going to bars and surely not being reclusive) for 30 years; it's pretty difficult to think it would not be one of the most known people on the island, given his role, unique appearance and the small community.
Stupidity: There was no particular reason for Nuclear Man to take Lacy into the space in the first place.
Stupidity: The whole movie happens for unbelievably convoluted causes. Despite their friendship and the simple fact that he still runs an occult shop (showing that he's not exactly insensitive to the past), Ray in years never spent a single moment to check on Egon's whereabouts, which he knew, or listen to him. The city has Shandor's name all over and any cursory investigation would have unveiled the connection. Assuming of course that Egon turned into such a lunatic he couldn't do the namedropping himself.
Stupidity: When the birds gather in the schoolyard, Melanie runs in to warn Annie. Rather than calling the police or fire department for help and then shelter in place until aid arrives (which would take about ten minutes), they instead decide to make a run for it with the children, out in the open, getting attacked by the birds. As it's the 1960s, the schoolhouse presumably has a working phone.
Stupidity: When the crew think that Dorothy is a robot with a bomb planted in her and is capable of exploding, they begin to think that she is about to explode and attempt to dodge it. Rather than hide somewhere or take cover behind an area that may protect them, they just get down on the floor in front of them and cover their heads with their hands. If they thought the explosion was going to be as deadly as the one they saw on the news, this method of taking cover wouldn't be even remotely effective. (00:24:40)
Stupidity: When Dr. James Harvey dies, there should be a body left behind in the manhole. After Kat and Casper revive him in his ghost form in the Lazarus, he comes out with a body (and clothing). One can assume that there are now two bodies of him, one alive and one dead. That would make an interesting article in the newspapers of the next day.
Stupidity: Given Maleficent's out there it seems stupid of the three fairies to leave Aurora alone in that bedroom.
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 Ruth goes to see Madame Arcati, after Elvira has arrived, the kettle on a gas two-burner hotplate is whistling, so Madame Arcati takes the whistle off the kettle and then the kettle off the hotplate and pours most of the water into a teapot, but, what is stupid is, she puts the kettle back onto the same burner without turning the gas off and in less than a minute the water is boiling its head off but they don't notice because the whistle is on the table and not where it is supposed to be, on the kettle.
Stupidity: The heroines guess the plan of the villain with a thought process that can only be explained by the fact that they read the movie script. There is no way for them to surmise that the attack on the Skrull base, perpetrated under subterfuge, was intended to "syphon away the atmosphere", and not just the usage of a really powerful weapon. Even assuming that Kamala's objection to this particular point ("they could have just invaded or used a bomb") is a logical explanation that would disqualify without a second thought the fact that a Kree warlord would use a terrifying and cost-free weapon that is also a symbol of Kree's culture just for its power, and so that it makes sense that the heroines just instantly commit to an alternative theory, there's still a leap of logic in assuming the exact next step for the Kree's quest on revitalizing their homeland (which incidentally involves randomly dumping out of the blue a gigantic quantity of salt water on a densely populated planet).
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: During the film's climax where Santa and his reindeer successfully ascend over Manhattan, the sleigh's rocket engine falls off and apparently is no longer needed. Isn't it rather risky (and foolish) to assume (or hope) that the other cities on his flight path have enough "belief power" to sustain him aloft for the rest of his journey?
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: When Jody traps Michael in his room, he does so by sticking a screwdriver between the the door and the jamb. This likely wouldn't accomplish much regardless, but it especially wouldn't work in this situation because the door opens into the room, so Michael could simply open the door and the screwdriver would fall on the ground.