
Stupidity: After all the singing and dancing at the Tent Revival, Lt Elizondo yells at her fellow officers "It's a Trick, they've brainwashed the Captain, get 'em." Considering her and everyone else just witnessed the Captain float into the sky surrounded by fireworks and miraculously change into a black suit her "brainwashing" assumption is truly stupid.

Stupidity: After leaving the church in the painted Studebaker, the duo have Max and Hopper right behind them. Therefore, there's no way the (conveniently) colored car could possibly fool Max and Hopper.
Suggested correction: That was the whole point. It was supposed to be funny.

Stupidity: When making apple sauce, just slapping a paper lid on the jar (instead of canning it properly) , would soon result in the jars turning to mold.

Stupidity: John Candy was being dragged by the speedboat because he was holding the bar when the boat took off. It simply never occurred to him that letting go off the bar would have solved the problem (Of course then you wouldn't have had the speedboat scene at all but it's still pretty dumb).
Suggested correction: Characters doing stupid things doesn't constitute a stupidity entry. That was part of the joke, that Chet spent all the time telling his son to remember to let go of the rope if something goes wrong, but then forgets his own advice in the heat of the moment. People do stupid things in real life all the time.
Well how does this not count as a Stupidity then? You just said it was and there was no need for him to stay holding onto the rope.
Stupidity is basically a minor plot hole, something small that doesn't rise to the level of an plot hole entry. Characters are still allowed to do stupid things though if it's not a plot hole (otherwise everything Lloyd and Harry do in all the Dumb and Dumber movies would be stupidity entries).

Stupidity: The Foot Clan knows that April O'Neil is an ally of the Teenage Mutant Ninja Turtles. Since the Foot Clan hasn't been able to find the Teenage Mutant Ninja Turtles for where they live a easy solution would be for them to go to April and ask where are the Teenage Mutant Ninja Turtles. An explanation is never given in the movie for why the Foot Clan never use that easy solution.
Suggested correction: First of all, the Foot Clan at the beginning of the movie is disorganized. Secondly, upon Shredder's return, he plants a Foot soldier named Freddy into April's news crew in order to keep an eye on her and find out where the Turtles are. Then they change tactics when they discover there is still some of the mutagenic ooze around, which they then decide to use to create their own mutants to lure the Turtles out of hiding.

Stupidity: When seeing that the bridge is out, JLC could've easily climbed out of the sunroof and hop off the back of the limo and onto the pavement, suffering a few cuts and bruises at the most. I know that it would've been anticlimactic, but still.
Suggested correction: You're assuming that jumping off a runaway car is a simple task that anyone can do. She's scared out of her mind, and many people in a similar situation would probably be scared at the prospect of jumping off a moving car, even with the knowledge that they will be flying off the road shortly.

Stupidity: Makes for a funny one-time gag for the benefit of the audience, but it doesn't really make sense that Jay would walk into the hitman's place without knowing he was deaf. It's not exactly the kind of detail that Tommy T. would not mention. In fact, it's the only thing he told the police, apparently, since they, on the other hand, show up knowing he's deaf but do not know he's a big ginger dude and are therefore initially fooled by his squatter story.

Stupidity: The Good Boys learned about consent in assembly and knew about making a citizen's arrest, using the internet to find where the drone is sold, how to treat a dislocated shoulder, etc. but did not know that they shouldn't give out their address on the internet and invite a stranger into the house?

Stupidity: In the bar scene, Jack gets into a fight with a guy he busted a few years back, then Reggie turns to the bartender and asks for a gun. The bartender hardly hesitates and hands it over. There is just simply no scenario where a bartender would hand a loaded gun to a patron they had just met.

Stupidity: Denny removed a tray of chicken/capons from the oven, set it on the counter, cut off a piece from one chicken and threw it to Enzo (who caught it in his mouth). If the meat had just been removed from the oven, it would have been too hot to give to a dog and Enzo could have burned his mouth/tongue. (00:51:25)

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: 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: After going through being stuck on a deserted island for 15 years and finally being rescued and brought back to civilization again, you would think if Skipper was going to sail out with the castaways on his new Minnow boat, he would pick an enclosed lake this time as opposed to the open ocean – now they got stuck on their island again because of that decision.

Stupidity: If the family is planning a big shopping trip, why would Clark still be in his morning clothes and busy putting presents in the attic as if he didn't know? And why wouldn't have anyone told him or reminded him before leaving the house? It just seems like a plot device using the foolishness of the characters to have him trapped in the attic.

Stupidity: When Steve Martin reaches the beach with the wheelchair, he tips it over, giving the illusion that he fell out of it. However, with where he starts crawling on the beach, it's tipped the wrong way. If it was that way, he would've been on the other side of it. A clever con-artist should know better.

Stupidity: Dr. Cocteau's choice to unleash an enhanced Simon Phoenix without any way to restrain him is incredibly reckless and stupid. Even if Simon were to kill Eager Friendly, in the best case situation, you'd still have a madman with total computer access, martial arts knowledge, etc., that you would have no way to rein in. Sure, he can't kill Dr. Cocteau, but what would stop him from say, holding the city hostage or something? Why not add in a kill code or something to keep him in check?
Suggested correction: Who says he didn't? Cocteau has put in mental conditioning compelling Phoenix to kill Edgar Friendly and make him unable to kill Cocteau. Who says he hasn't put in something that makes him kill himself after the deed is done? Or perhaps paralyze him so he can be put back on ice. It's just that Cocteau didn't count on the fact that his henchmen could kill him. He doesn't care about how dangerous he is, not until he has done the deed.
Dr. Cocteau is a narcissistic egomaniac type that would see himself as a king or a god, even. And Simon is making him very angry. He even tells Simon, "you're beginning to be more trouble than you're worth..." Someone with an ego like Cocteau wouldn't stand for Simon's antics for very long. And would happily enjoy putting Simon back in his place by shocking, paralyzing, etc.
But he first needs him to kill Friendly. Until he does that, he'll let him play. He still sees no danger to himself.

Stupidity: Darren could have easily grabbed Ben out of the locker he climbed into by just pulling. Instead, he pushes it inward, causing it to close and allowing Ben to either hold onto it on the inside or lock the locker itself.

Stupidity: Luca's parents try to find him by splashing him with water. If they succeeded, Luca's cover would have been blown.