Stupidity: After the accident, Dawn is in spirit form on the road trying to wave down an oncoming car (driven by Martha Kent). Dawn doesn't know she's a spirit that can pass through objects at that point. Since Martha can't see her, she keeps driving straight towards Dawn. Dawn sees the vehicle isn't slowing down and instead of simply stepping out of the way to avoid getting hit, she turns her back, revealing to the audience that objects can pass through her now.
The Tale of the Dream Girl - S3-E10
Stupidity: A boy walking through the bowling alley stops and lets Johnny pass, even though Johnny is supposed to be a ghost no one can see.
Stupidity: In the flashback, Emily's sister takes ill while playing her flute. In the next scene, the sister is sick in bed and Emily now has the morpher around her neck, but she is in the exact same outfit. If the sister was forced to pass the legacy on due to disease, it should have taken more than just a few minutes or even hours to learn that.
Storm Before the Calm: Part II - S1-E38
Stupidity: Shane could have easily avoided the blast from Lothor by simply flying away to dodge it. Instead, he just hovers over him and allows his Battlizer to be destroyed.
Stupidity: After Bernie discovers a treatment for the infection, radiotherapy, everything goes back to normal. Except the contagion is airborne (which is why they were going to flush the entire ship's atmosphere), so they're just going to re-infect themselves in a few hours. And there's only one room on the ship that can be exposed to stellar radiation, so there's no way to clean the atmosphere either.
The Night of the Firebrand - S3-E2
Stupidity: West is introduced to, then knocked unconscious by Vixen as she hides a club behind her back. A trained Secret Service agent like West would not have let his guard down in this situation, as the entire fort was awash in bad guys.
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?
Recognition - S1-E17
Stupidity: After fighting Wootox in Sky's body, why didn't Cruger contact the other Rangers and inform them about the body switch? This would have spared everyone a lot of trouble.
Emohawk - Polymorph II - S6-E4
Stupidity: On the walk back to Starbug, the Emohawk jumps on Lister's head to disguise itself as Lister's hat, but it leaves itself in its natural form whilst the crew continue to walk. The Emohawk is supposed to be disguising itself to fool the crew into making them think it is any everyday object. The Emohawk eventually changes into Lister's hat, but leaving itself in its natural form even for just a small amount of time could blow its cover. (00:18:15)
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.
Worlds Apart - S1-E2
Stupidity: The bad guy kills the deputy in the swamp, then later sees a little girl who witnesses the killing run away. He then searches the boat for a weapon and runs off after her after he finds a machete, without even considering the officer's gun still in its holster. Abby proceeds to make the exact same mistake later when she finds the dead officer (meaning there's danger nearby) and hears the girl screaming and runs in the direction of the screaming without taking the gun to protect herself.
Through the Valley of Shadows - S2-E12
Stupidity: Burnham and Spock find one person alive from the otherwise-dead crew, miraculously. They know Control can infect humans, they know how advanced it is, the entire setup screams "trap", and yet they don't even question whether Gant even might be under Control's...well, control. A simple scan would have turned up anomalies and they could have avoided what followed.