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.
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)
The Alchemy Murder Case File 4 - S1-E9
Stupidity: Everyone mentions in the first episode that the various doors are made out of different metals (some rather unlikely; a door made out of pure manganese?) but they don't show it, all have the same color and weight (the door made out of lead and the one made of tin would be significantly different, and one is the treasure, 966 kg). More importantly, since it's a known fact that the doors are made out of different metals, each has a number that matches the periodic table, and they are looking for gold, the supposedly hard enigma concocted by the genius alchemist is something a high schooler would figure out in minutes; Miyuki even knows its atomic number by memory when randomly asked.