Crisis on Earth-X, Part 3 - S4-E8
Stupidity: After the power cuts out, Thawne realises an obvious hiding spot and races to the secret room. He sees it's empty and just...stops looking? We literally just saw The Flash take a fraction of a second to scout out an entire enemy base we're told was 22km away! While he's dawdling in the secret room, Iris and Felicity get to Kara, knock out the guards, and get her down the corridor to the lift. He could have checked the entirety of Star Labs 10 times over in that time.
The Prison Prep School Murder Case File 2 - S1-E11
Stupidity: The whole story of the 5 episode arc requires unhealthy amounts of suspension of disbelief; the killer directly challenged the police and announced where and when the murders will take place; at a 3 days prep school camp. Despite that, one of the teachers, a new one at the job even, is allowed to attend the camp wearing a ski mask, supposedly to hide hideous burns, and the police just lets him in without checking. And yes, he's the criminal, not even a red herring. By comparison, in the live version adaptation they check him, and he's wearing prosthetic scars.
Stupidity: During a raid of a Latin stash house, Hailey and Kim find two girls with a pistol. In Spanish, they say no bullets. Kim interprets what the girls say and Hailey replies, l didn't know you spoke Spanish. Makes no sense that a partner on an elite police unit in Chicago would not know a member of the team speaks Spanish. (00:14:58)