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.
It's Only Rock and Roll - S3-E11
Stupidity: When the gangsters show up at the house Pam and Dak Hampton are hiding in, Pam is captured. Dak then attempts to flee by running out the front door. What a dumb move. No surprise that he got captured as well.
Who, What, Where, Wendigo? - S2-E10
Stupidity: Dwight, Audrey, and Nathan all know Dwight's trouble is that all bullets are attracted to him, so why is he with them when they go looking for the killer, knowing every one is using a gun?
In a Mirror, Darkly (2) - S4-E19
Stupidity: When the Gorn attacks Archer, one of the MACO officers raises his weapon to fire, but fears hitting Archer, so he doesn't shoot. But then he attacks the Gorn with the butt of his weapon. He was close enough to hit him, he could have just stood over him and fired without fear of hitting Archer.
Stupidity: The Tomorrow People tend to enjoy walking into and walking out of most scenes rather than teleporting. In particular, when fighting and needing to run away, they'll teleport to get a better fighting vantage instead of teleporting out of the situation.
Stupidity: During the scene at the restaurant, Rose pulls the fire alarm. Then she and the doctor run through the kitchen, past several chefs who just duck and crouch into corners. This would easily be one of the stupidest ideas ever. If there was a fire, they would evacuate out the back door instead of just sitting there waiting to be choked to death. And if they knew about Auton Mickey, running out the back door would also be a lot more logical than just crouching down and waiting to be battered to death.
Stupidity: The Zeon commandos who plant the bombs on the Gundam correctly identify Amuro as the pilot. The Gundam is completely defenseless and pilotless during the whole bomb removal process - they could attack at any point and make the bombs explode shooting them like they did for the first one, but they just don't.