Stupidity: When the trolls are trapped in Chef's cage, Smidge is small enough to slip through the bars and could've easily escaped.
Stupidity: The Engineer and Fireman of the Polar Express train crew should have known better than to not bend back the the metallic prongs of the cotter pin, which would have prevented the cotter pin from coming loose and causing all those problems in the driving of the train.
Suggested correction: As you yourself stated, the whole thing is likely a dream, where "normal" reality doesn't apply. In the "real world", the train would never be able to do any of the things that it does in the film.
Dream or not, it is still a stupidity of the train crew to not secure a pin that could work itself free of the controls of the locomotive.
Dreams are often unrealistic. There is no mistake.
The entry doesn't say anything about the entire trip to the North Pole being a dream.
Stupidity: During one of the test drives, the hook that Hiccup uses to fasten himself to Toothless gets bent, so he takes Toothless to the blacksmith to cut the line. There are two ways he could have easily avoided this: He could just unhook the saddle, take that to the blacksmith and cut himself loose, or he could just untie the leather strap. What he does here is needlessly risky and just there for his awkward conversation with Astrid.
Stupidity: When Jasmine finds out that Prince Ali is actually the boy she met in the marketplace, she seems to completely forget about Jafar telling her he had been executed. She never asks why he's alive, nor does she even bother to confront Jafar about his treachery.
Suggested correction: The fact Aladdin was alive only told her Jafar lied about it. When she is brought back by Aladdin to the palace it is still night, not long after that she is confronted by her father and Jafar who has him under a spell. She hardly had time to ask about the treachery, probably thinking to do it in the morning. It is there and then Jafar is exposed by Aladdin. She probably wondered about it, but didn't figure out the implications yet.
Stupidity: Al is supposed to be a super serious toy collector and seller, but he handles valuable toys without cleaning the cheese puff dust off his hands.
Suggested correction: It's also established someone is coming over to clean said toys, dust included, and when he does the handling, he's just been sharply awakened by the TV therefore is off-kilter.
Stupidity: Stitch wakes up in a kennel with dogs at the animal shelter. When Lilo picks him out and they walk out of the kennel, the lady freaks out and tells Nani, "We thought it was dead, it was hit by a truck!" So, what was thought to be a dead dog was put into a kennel with other living dogs. Think about that for a second. (00:24:54)
Stupidity: Long after the Marine Life Institute has closed for the night, the recording of Sigourney Weaver welcoming visitors is still playing.
Stupidity: For some reason, Roxanne didn't realise Tighten was Hal until after he took off his mask. Nor did she recognise Megamind's voice when he was disguised as Bernard.
Stupidity: While being pursued by the Headless Horseman, when Ichabod first approaches the bridge, he could have gotten off his horse and fled on foot.
Suggested correction: Considering that the Horseman's horse is supernaturally fast, if Ichabod had gotten off his horse and ran for the bridge, the Horseman definitely would have caught up to him.
Stupidity: Captain Hook has a ship full of cannons and other weapons, but he never tries to kill the crocodile trying to eat him. Why doesn't he just kill the crocodile?
Suggested correction: Setting aside the fairytale nature of the story, in which logical holes like this should be overlooked, the crocodile is not available at Hook's convenience, and Hook is too terrified of the croc to seek it out.
Stupidity: Luca's parents try to find him by splashing him with water. If they succeeded, Luca's cover would have been blown.
Stupidity: Jimmy and his friends aren't allowed by their parents to go to the opening of Retroland because it is a school night. What kind of amusement has its grand opening on a school night? It would make more sense commercially to open on a Friday or Saturday night.
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.
Suggested correction: You don't think that once Nebbercracker noticed the house had the spirit of his wife, he would have realised this could happen one day. In order to protect her, he most likely blocked off the fireplace.
Stupidity: This film portrays Michael Jordan and his family living in a relatively modest house in the suburbs. Jordan was one of the most famous people on the entire planet around the time the film takes place. Living in such an area would lead to a never-ending onslaught of overzealous fans, paparazzi, and even detractors.
Suggested correction: We see Ariel sign in English, but most likely they have an entirely different alphabet and language from our own.