Stupidity: After the Slick devours Laverne, Randy jumps into the water and swims for shore with the Slick following. Even though Randy made it to shore, he stops and turns to confront the Slick, screaming that he beat it. When Randy turns around, the Slick immediately envelopes him. If Randy had just got up and started running instead of facing the Slick, he would have lived.
Suggested correction: This is a deliberate movie making technique to make the audience think the character has escaped but at the last second a surprise is thrown in. It's the same as hanging over the body of the killer. It doesn't come under stupidity.
Yes, it does. In any movie where someone comes face to face with either a supernatural enemy or even a regular one, confronting it is extremely stupid because the character could have simply walked away and made it to safety. The character confronting the killer is stupid because they'll always be killed. Better to do the smart thing and run as far and as fast as possible.
A example of a valid stupidity entry is an astronaut taking his helmet off when in space because he's an expert and knows better. A stupid person doing something stupid is considered a stupidity entry (which is essentially a plot hole writers use to move the story in a particular direction it wouldn't have gone otherwise). A character making the wrong choice because they underestimate the situation isn't a minor plot hole, in real life people underestimate opponents all the time, and movies exploit that all the time in their plot development (i.e. Apollo underestimating Rocky).
Suggested correction: Randy's decision isn't what constitutes a "stupidity" mistake. Stupidity mistakes are minor plot holes, which means characters can act stupid. Plus, when Randy gets to shore he's worn out so he tries to catch his breath. He then says he's won because he under estimated the Slick's ability to get him (which would fall under the category of "celebrating too early").
Put yourself in Randy's place. If you're friends were eaten by a huge slick like monster and you were swimming to shore with it chasing you, after making it safely to shore, would you want to turn around to confront it. No. That would be stupid because confronting it will certainly get you killed the moment your back is turned. The smart thing to do would be to keep running. Randy facing it was very stupid. Had he done the smart thing and kept going after he made it to shore, he would have survived. Stupidity killed him. Pure and simple.
First off, you said it would certainly kill you once your back is turned, which means if you're running away, your back is turned and if you confront it, your back isn't turned. But, he never tried to confront it. He just celebrates beating it. However, Randy thought he was safe once on land because he thought the creature couldn't attack him or reach him, so in Randy's mind he wasn't doing anything stupid. He thought he was safe, he thought he won. He was tired and sat to rest. But that's part of his character and his character traits. But, acting stupid isn't a "stupidity" mistake. Otherwise movies like "Dumb and Dumber" would just be thousands of stupidity mistakes because stupid characters are acting stupid. Now, if Randy knew the creature could kill him in water and he turns to celebrate his victory, or stops to rest, in the water, that could be a "stupidity" mistake since his character was already shown to know he can't stop in the water and the writers ignored what was already established as his character. Stupidity mistakes just are minor plot holes that go against already establish character traits or established facts/statements in the film.
Not only that, but, Randy was acting stupid since he chose to face it rather then run.
If you re choosing to say its a stupidity because they choose to fight rather than run then that's a moot point because you wouldn't have a movie in the first place! Movies get a degree of latitude when it comes to reality (people can be shot 7 times and still walk away as the credits roll) so rather than stupidity, this is under slightly suspended reality of how an actual person would behave. In this segment, we're talking about an oil slick with a mind of its own. 100% reality has to take a back seat where plot forwarding is concerned.
Stupidity: There was no particular reason for Nuclear Man to take Lacy into the space in the first place.
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.