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.
Reika Hayami and the Uninvited Guest - S1-E5
Stupidity: The plot hinges on the fact that the culprit switched clothes with the guy in the bathroom. It would be unfair to say that they do not address the issue, since both the shoes and the belt buckle are mentioned. However, this can work just in a cartoon, since he could wear without any issue and looking absolutely dapper the jacket and shirt of an obese guy. Anyone trying that in reality wouldn't just have to tighten the belt.
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)