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.
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.
Episode #1.7 - S1-E7
Stupidity: When Eugene approaches Tsuda after Tsuda is shot by Ae-sin, Tsuda attempts to shoot him, but after pulling the trigger, he realises there are no bullets in the gun. Seconds later, a group of soldiers run up to him, and he attempts to shoot them, but he can't because the gun is still not loaded. Something he should have known since he only just attempted to shoot Eugene and failed because of it. (00:50:00 - 00:50:30)
Stupidity: On his own in his helicopter, TC earlier in the episode spots a very hard to catch area of vegetation where supposedly the color of the leaves is different from what's expected. However for the ending, with 4 people in the helicopter, everyone but Magnum misses a giant clearing with a bright red excavator in the open.