Stupidity: Amazing how people on this show, like The Accused here and in the earlier episode "The Chef", are leaving their doors unlocked or no explanations given as to how the actual killer planted the evidence.
Stupidity: The inspector knows that his house has been bugged; he does not know how many bugs and cameras are there, he just knows there are some. He does find a bunch of them with great ease when he gets back home - making you wonder how could he have ignored them to begin with, since all it takes is for him to raise his eyes, they are fairly big black cameras who do not camouflage at all for the most part. He then proceeds to make a phone call to the big baddie from his landline. Nobody in their right mind would do such a thing. Heck, you wouldn't want to use your home phone for such a call even if you didn't know that someone just put bugs all over your house.
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.
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.
Stupidity: When Brass and Sarah enter the locked storage cellar where the boys are hidden, Greg, a graduated rookie, straggles along behind them when he should have known better to stay put. As anyone could have come along and locked that cellar door behind them, trapping them inside.
And All Through the House - S1-E2
Stupidity: The woman could've blamed her husband's death on the maniac, she didn't need to go outside to retrieve his body.
Camp Redwood - S9-E1
Stupidity: Brooke leaves her apartment window open as she sleeps, even after telling her friends that serial killers (like The Night Stalker) strike during the summer when everybody leaves their windows open.
Stupidity: As far as I can tell this is not a problem introduced by the novelization, but already coming from the original story: without Poirot's involvement, called upon by the murderer, the police would not have suspected murder at all, and still would have a witness with a rock solid alibi to talk about The Dream. If they really wanted another witness, they could and should have summoned a psychiatrist and do to them the same stage act they did with Poirot, they would have been much more qualified witnesses to frame it as suicide. Even to Poirot himself, it's the murderer who suggests the thought there could be foul play involved, at all! The plan makes zero sense because Poirot is not the ideal witness and they want to suggest the victim was mentally ill and suicidal, not that someone wanted to kill him.
Chapter Eight: The Battle of Starcourt - S3-E8
Stupidity: SPOILERS AHEAD: It has been a good minute between Hopper killing Grigory and Joyce turning the key. He could have used that time to run as quickly as he can towards her, instead of staring at her for 30 seconds. Also, Dustin didn't have to yell (through the radio) so panicky yet about closing the gate. While Mind Flayer was killing Billy, El and other kids could have used the time to run further again and get another 2-3 minutes, in that time Hopper would surely come to a safe place.
The Prison Prep School Murder Case File 3 - S1-E12
Stupidity: When Kirisawa's body is discovered, it means that he was completely overlooked during the headcount and at every point during the day. With students disappearing left and right, that's a pretty big oversight - that somehow the killer anticipated, because his body discovered last was crucial for the plan.
Stupidity: The brothers are angry at Leo because they say he's responsible for them dying because he chose to save other soldiers instead of them, Leo also blames himself for the same reason and says he should have stayed and gotten them out. But in the flashback it shows Leo helping them and then running off to help other soldiers, after he runs off the brothers are killed just a few seconds later. Even if Leo had chosen to stay with the brother he couldn't have gotten them out in time, the only difference would have been Leo would have died with them, but Leo dies in the same war so he didn't live much longer. Leo's guilt and the brothers' blame don't make any sense at all.
A Rough Whimper of Insanity - S1-E7
Stupidity: The hacker's VR goggles are not plugged in to anything (despite having a wire for this purpose) when he and Tom sit down at the computer in the teacher's house.
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?
Stupidity: S1E8 Obsessed - Morgan and the detectives respond to the beach for a possible homicide. None of the detectives or Morgan are wearing protective gloves. Throughout the series, the detectives remind Morgan to wear gloves before touching evidence. However, she takes the pulse of the victim without gloves. (00:04:02)
Is Lupin Burning... ?! - S1-E1
Stupidity: Despite the fact that there are 100 (literally) cameras around the racing circuit and (again literally) everyone from the staff to the fellow racers works for the Scorpion gang, Jigen and Lupin's switch can happen without anyone noticing, even if it involved racing side by side for a lengthy period of time.
Day 4: 11:00 P.M.-12:00 A.M. - S4-E17
Stupidity: When the couple is in the plant they call Jack on the cellphone, he instructs them to open the case and separate the book from the activation device. This was stupid, he should have told them to destroy it, since the codes had changed the moment Air Force One crashed, and it would take about 1 hour to make this work. This is explained later on in the show.