Stupidity: When Lois, possessed by Dawn, comes downstairs and says she's going to the prom and Clark is taking her, Clark doesn't think for a moment it could be Dawn. He already knows Dawn can posses others, those people don't act like themselves, Dawn is obsessed with the prom, Lois isn't acting like herself, and Lois is now not only suddenly prom obsessed for no reason, but wants to go with Clark when she said that would never happen.
Episode #1.8 - S1-E8
Stupidity: There's a gunman that just opened the school doors, and he's looking at the police like "a deer caught in the headlights" and they are all pointing their weapons at him and he's holding a gun mind you, but somehow these cops let him run back into the school without even taking a singe shot at him. His background was clear too, so if the cops fired there would've been no collateral damage if they had a shoot out. (00:35:00 - 00:35:30)
Episode #1.2 - S1-E2
Stupidity: The part when the Colonel and the governess pose as journalists does not make sense. It's far-fetched enough that after a lengthy interview about the culprit's fortune the topic of his family never came up, considering it was one of the main goals and an ideal question to lead with, but considering that "mr. Todo" does not want a picture taken and has a tight security, there's no way that a photographer carrying a large 1930 camera would have been allowed into the interview room to begin with.
Backlash of the Hunter - S1-E1
Stupidity: Rockford knows almost nothing about the guy who is tailing him except that he noticed him while he was grabbing a bite with his client. Regardless of that, he sends the same client to seduce him, just assuming that he won't be able to recognize her. And it works. Is Lindsay Wagner this hard to forget, that this guy just an hour or two later isn't able to tell it's the same woman his target was meeting?