The Book of Rebellion: Chapter Three: Angelitos Negros - S2-E10
Stupidity: The SWAT van transporting Khalil comes upon the most obvious setup in the history of setups, a crashed car with a miraculously unharmed woman lying in the street next to it, one arm suspiciously out of view. And of course one highly trained SWAT agent, instead of going in with backup, circling around for a proper look, or driving around (as his teammate suggests) or anything similar, wanders over and is immediately killed for his trouble. And then of course despite being surrounded by armed cops, she somehow manages to kill all of them without a problem (offscreen of course, because there's no way to believably show her single handedly killing about 20 cops).
Stupidity: Donna runs in to save everyone from the collapsing pylon, dying in the process, despite the fact that Superboy or Raven, both more powerful/resilient, could have stopped it falling without any issues whatsoever. Not even like anyone was distracted - the fight was over, they were all just standing around.
Infestation - S1-E3
Stupidity: So "Dr. Smith" runs into her sister's boyfriend on the Resolute and pushes him into an airlock when he figures out who she is. The door immediately closes and a countdown to purge the airlock ensues. Who would design an airlock to do that? What if someone went in there by mistake or was doing some maintenance on it? To make it worse, there's no override inside the airlock? It's located in the corridor.
Stupidity: "Rei" comes into the circle, having been champing at the bit to "turn him into a smear on the floor." She's wielding a sword, Kovacs is unarmed, and she...slices him a bit, instead of a quick killing blow.