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: First, the hunter ties Benji with a nearly 10-metre-long rope - how did he expect that the dog wouldn't trash his place like that? Second, after the wolf attack, he simply unties Benji and puts him right next to the tied bird, serving the dog the chance to grab and escape with it on a silver platter.
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: Central to the plot is the layout of the Mars lander. There are no seats or any restraints of any kind in the lander, which is ridiculous. Spacecraft require safety belts in order to prevent astronauts from being injured during rapid course corrections. Additionally, ordinary rolling desk chairs would not be sent into space, and would certainly not be placed on a landing vehicle.
Stupidity: When Bob is fighting one of the Russians, Tatiana could have kidnapped Nota but instead stands beside her and watches. (00:52:05)
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: Just after the geckos have finished eating Josephine's children, Thang, Rose, Didi and Sal attempt to sneak away. They attempt to sneak away from the geckos by walking directly in front of them which results in the geckos spotting them. If they attempted to get away in literally any other direction, the geckos would not have seen them. (00:22:50)