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: If the fairies knew that Maleficent was going to try and kill Rose on her 16th birthday, why, oh why, do they take her back to the castle on that day? Why take her back to where Maleficent was expecting her? Why not wait until after her birthday?
Stupidity: At the end of "You're Welcome", Maui puts Moana in a cave and placed a large rock blocking the only known exit. This means, as far as Maui knows, he's trapped a human in a cave with no food, water or exit as he didn't know Moana could make a very lucky escape. Basically Maui was going to leave Moana to starve to death so he could take her boat.
Suggested correction: This isn't a mistake. Maui doesn't care about anything other than escaping at this point. He even says, Without his hook he is nothing. He's not interested in returning the heart at first so even though he is a demigod, he has lost interest in being good. Another example is when he leaves Moana to face TeKa on her own.
Suggested correction: We see Ariel sign in English, but most likely they have an entirely different alphabet and language from our own.