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.
The Woman Who Never Died - S1-E3
Stupidity: The detective Magnum is with does not call reinforcements because they "might spook" the criminal, but he arrives on the scene making a lot of noise and with the emergency lights on. Even if the reinforcements arrived in police cars, they could have easily kept out of sight of the house and lurk around the block.