Stupidity: Dr Kepple is finally caught when he rushes to the hiding place of the calibration converter that he hid in the lamp after the murder. Why is it still there? It was a great hiding place initially when the police were searching people and the building in the hours after the murder but Kepple had plenty of opportunities after that to get rid of it. It is the only piece of evidence linking him to the crime and it is in his office so you would think that his main priority would be to dispose of it as soon as possible.
Stupidity: Why would a cop bring up an old case when he knew he was the murderer? It makes no logical sense and only brought his downfall.
Suggested correction: He was being blackmailed by Gary Roberts. He "reopened" the case as a ruse to kill Roberts.
Stay Out Of The Basement (2) - S1-E13
Stupidity: When the real Dr. Brewer is found in the basement, he has duct tape over his mouth and needs his daughter to remove it for him. His hands are tied around the wrist, but not behind his back, so he could have easily removed the tape from his mouth himself long before he was found.
The Return of Hydro-Man: Part 1 - S5-E7
Stupidity: When Spider-Man is working in the lab, he uses his webbing to barricade the door shut so no-one interrupts him. When Debra Whitman shows up and asks who is in there, he identifies himself as Peter Parker. When he is finished with his work, he leaves out of the window so that by the time the security guards break the door down, he has vanished. Since he used his webbing to keep the door closed, it would be pretty easy for Debra and the security guards to figure out that Peter is Spider-Man since Spider-Man's webbing is at the scene and Peter seemingly vanished into thin air.
Episode #3.1 - S3-E1
Stupidity: Ronan Murphy is left handed. Watch carefully as he disarms himself. His weapon is replaced in his right hand. In a suspicious shooting, they never thought to check something so basic? (00:04:12)
Stupidity: After talking to the bartender, Calleigh goes back the lab and talks to Caine, and he wonders if they got a false positive from the vic's blood alcohol. Calleigh then gives a detailed description of how that could happen; Caine already knows how (obviously this was done for the audience, to explain it, but in the context of the show, it makes no sense).
Stupidity: When Mac and Lindsay are reconstructing the stab wound found on their victim on their pig specimen, a couple of the weapons they use are long, thin knives; since they obviously don't match the circular wound, Mac at the very least would know those could be excluded by default.
The Promise - S6-E13
Stupidity: Markham lets Ava call her uncle's phone, and she doesn't let on that Boyd answers instead. He later says "I don't trust anyone", but he didn't demand she put the call on speaker for transparency, he's quite happy (at the time) to just hear one side of the call.
The Forecaster (No. 163) - S4-E10
Stupidity: Farnsworth tells Charnquist they've been compromised by a girl listening in on their phone calls and that Charnquist needs to take care of it. He is able to identify the girl, elude FBI, tries to burn his apartment down, is able to gains access to Maggie's apartment and knock out Aram in order to kidnap Maggie. He's a killer who works for someone who has arranged dozen of murders, and knows "take care of her" means kill her. There is absolutely no reason for Charnquist not to have just killed Maggie, Aram, and Fiona. He had opportunity and he ends up killing 3 cops in order to kidnap her and kidnapping Maggie would not have solved anything.
Stupidity: During a shootout in front of a bank, one of the bank robbers lays his shotgun on the trunk of a car and yells "I'm out!" However, he has 6 additional rounds in a shell holder on the side of the gun. (00:35:20)
Episode #1.3 - S1-E3
Stupidity: Julie took Sean's phone, and we learn she cut his thumb off so she could keep it on her to unlock the phone whenever needed. This makes for a funny moment but makes zero logical sense - having unlocked his phone once, she could just remove his thumbprint and use her own instead, or disable the security entirely.
Mr. October - S1-E2
Stupidity: The two men with Hermit decide to jump down a floor into the damaged building since the stairs are gone. They're aware the structure is unstable and the ship inside is dangerous, but leap without proper rescue gear, trapping themselves with no way back up. For a supposed rescue mission, it's reckless and leaves them stuck if things collapse. (00:10:45)
Stupidity: When the new principal is addressing the student body, Avalanche tries to impress Kitty by causing a couple of small earthquakes, one of which causes the scoreboard to crash to the floor below. The principal laughs off the incident and continues to address everyone instead of either evacuating the area or having everyone take cover.
God Complex - S5-E23
Stupidity: Maggie and OA are searching the building. As they walk up the stairs, Maggie is using the Harries technique, but she has both her handheld flashlight and the gun's flashlight on at the same time. This defeats the purpose of the technique. Additionally, it's daytime, so the building is lit up from natural light sources, but they still have their flashlights on. (00:13:36)
Stupidity: After Dexter tells Hannah that the U.S. Marshals have distributed her picture everywhere, she questions how she will be able to flee Miami for Argentina. Dexter reasons he and Harrison can leave with her because the authorities won't be looking for a woman traveling with her "husband and child." It doesn't matter who she is or isn't traveling with. If the authorities have shared her picture, she's going to be flagged when she goes through security at the airport, which Dexter should certainly know. This is made all the more egregious when she takes Harrison to the hospital later in the episode and is ID'd by the receptionist.
Will the Real Martian Please Stand Up - S2-E28
Stupidity: The whole plot is based upon 6 people not being able to spot the seventh (odd man out). We can eliminate the couples, the driver remembers the pretty blond getting on the bus and the crazy old guy would stick in anybodies memory, so it leaves only the sour old guy in the coat and hat as the martian.Also all the troopers has to do was ask for IDs which they do but quit after they ask the crazy old guy for his.
Stupidity: One of Rogue's defining character traits is her inability to have physical contact with another person because of her powers and the sorrow it causes her. Several episodes of this show, however, featured devices that could suppress mutant powers, the inhibitor collars being the most prominent. Despite this technology at their disposal, they never even think to have Rogue wear an inhibitor device for times she would like to touch someone. Even if she didn't want to wear those specific collars, a character like Beast or Forge could easily create a new and harmless device by reverse engineering and adapting the existing ones. This of course would eliminate Rogue's ongoing dilemma and potentially curb her development as a character, but it doesn't make sense from the standpoint that she longs for physical contact with others.
Stupidity: The experienced cop Pullman enters the house of the well-known badass villain with her partner only, without any more backup. After the partner gets killed, Pullman stands free in the middle of the room without any cover - just 'begging' to get shot dead. (00:53:15)