Stupidity: When Lawrence begins turning into a werewolf in front of Dr. Hoenneger, his fellow doctors, Inspector Aberline and the guard, everybody just sits in their seats and watches the transformation instead of running for the door.
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 the bellhop is in the elevator and it looks like this is where he loses his arm, he spends a few minutes trying to stop the elevator. All he had to do was let go of the luggage and he would have been able to easily bring his arm in.
Stupidity: Why in the world would Tony of all people leave his old suits in storage WITH the miniature arc reactor still inside the suit? This would have served no purpose to Tony since he already has his own arc reactor in his body.
Suggested correction: This is a question which is easily answered; Tony had the idea of putting someone else in the suit (like Rhodey) himself. Plus, it's logical to think he has built multiple miniature arc reactors, after the events of the previous movie. Plus, he can probably just as easily switch from arc reactor to arc reactor in his chest when changing suits, so he has a fully powered arc reactor ready in each suit.
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: Cobb convinces Fischer to enter a dream saying it's Browning's dream. A couple of scenes later he says it's Eames' dream, in front of Fischer, who doesn't know Eames was impersonating Browning. (00:51:35)
Suggested correction: Firstly, the Browning that the team "catches" is not Eames, it is a projection of Fischer's subconscious. Eames was only impersonating Browning in the first level. Secondly, Cobb doesn't say they are going into Browning's dream, he says they should put him under and do to him what the kidnappers were planning to do to Fischer. That is, use Fischer as the subject of the next dream level. Fischer knows that Eames is the dreamer on the third level, he even jokingly asks why Eames couldn't dream of a beach. But he thinks that Eames is part of Mr. Charles' security team. He believes that Browning is to be the subject on the last level when in fact, Fischer is the subject on each level. Because of how effective the Mr. Charles gambit was, Fischer is unaware that the team has been fighting his subconscious all along.
Stupidity: When Harry finds the Sword of Gryffindor at the bottom of the frozen pond, he breaks a man-sized hole in the ice, then strips down and jumps in. He could have broken a far larger hole in the ice, negating the risk of being unable to locate the hole upon coming back up. But because he doesn't, he nearly drowns when the locket resists and he can't surface (he couldn't have anticipated the locket's resistance, but that such a small hole would be hard to get out through was easily foreseeable.)
Stupidity: Jake throws Derek overboard to distract the piranha, then swims under the boat and doesn't get attacked, so why couldn't he just go downstairs the normal way and have Kelly swim over to him? They could have escaped a lot more easily.
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: Noah chases one suspect while Leslie chases the other two. Noah eventually pulls out his gun and says, "Stop, police!" Leslie doesn't even try to draw her weapon until she hears sounds from the nearby brushes - but she barely has her hand on the holster when she turns around and gets punched in the face. Leslie is knocked over and the other man she was chasing repeatedly hits her in the face, quite hard. Her injuries could have been avoided simply by having her gun ready to aim and use. (01:04:48)