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 protagonists need to know what Pope is planning with his drug shipments, in secret. So they sneak under a false name into Pope's party and... the policewoman goes dancing with Pope, who knows her and that she is a police officer! Good that nobody asks her anything - she'd have no way to explain her presence! At no point it is even hinted that he was just about to go upstairs, why would he, after all, with all those guests? And their secret mission continues with the alarm being triggered and Ice Cube KTFOing guards. Some stealthy mission, and they are even captured at the end! And even after all these epic fails, they just assume that Pope is not going to change anything about the shipment details. And to make all of this even worse, they know about the port commissioner situation, they talk about it and how necessary it is for Pope to control the commissioner, but don't take half a second to mention his name, which is all it takes for AJ later in the movie to 'remember' that he's been bribed.
Stupidity: In a flashback scene, a mother, father and daughter are taking pictures of Norm and two other polar bears. The daughter decides to run up to the three polar bears, but the parents appear to do nothing about it. The polar bears could have been a threat to their daughter for all they knew. (00:04:25)
Stupidity: Long after the Marine Life Institute has closed for the night, the recording of Sigourney Weaver welcoming visitors is still playing.
Stupidity: Bridget car keys are lying on the shelves (again) after she has left her bags at the ATM. I suppose they could have taken her car after all when she and Mark try to think of a way to get to the hospital.
Stupidity: The Joneses forget to change their New York license plates to Georgia ones as part of their cover.
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.