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.
Blood Pool Hall Murder Case File 2 - S2-E6
Stupidity: Even assuming that running in flipflops down the gravel road would give the culprit the required advantage over Kenmochi and Kindaichi, the culprit still had to run the elaborate setup in an absurdly short amount of time without leaving behind a single piece of gravel or wet stain (he was elbow deep in water) and hope nobody would set foot in the courtyard during the commotion.
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)