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.
A Deadly State of Mind - S4-E6
Stupidity: In the final scene: Columbo lays a trap for the murderer (George Hamilton). The 'eyewitness' when Hamilton fled the murder scene was a blind man. Hamilton is aware of this. Columbo's gambit depends on Hamilton's assumption that Columbo has coached the witness to act as though he was sighted so as to convince Hamilton that there is evidence against him. In fact, Columbo has the witness' brother, who is sighted, pretend to be the eyewitness and identify Hamilton as the man he saw driving away. When Hamilton falls for Columbo's trap and, believing he is in the presence of the blind man, tries and fails to 'prove' the man can't see. By doing so he reveals his knowledge that the witness was blind, incriminating himself. The problem is, that only a minute or so previously, Columbo had placed Hamilton under arrest, and failed to read him his Miranda Rights. Considering that Hamilton's slip up is the ONLY evidence Columbo has against Hamilton, this was a spectacularly stupid move on Columbo's part. Regardless of how many officers were present to witness this, the fact is that Hamilton was under arrest and had not been advised of his right against self-incrimination; meaning nothing that he said when under arrest can be used against him.
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?
The Kidnapped Prime Minister - S2-E8
Stupidity: In the episode, Hastings stalks Mrs. Daniels, being parked for hours literally in front of her windows, and the army surrounds the destroyed family mansion 'hiding' behind bushes totally visible, not scattered and standing in a way that makes them perfect targets. Nobody knows the first thing about secrecy.
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.
Camp Redwood - S9-E1
Stupidity: Brooke uses a flashlight to enter the cabins rather than turning on the cabin light switches. The camp has electricity powering the lights, TV, etc.
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: The gas station attendant can (at least hypothetically, he turns out to be wrong) recognise someone who did a hit-and-run a long distance away at night, but somehow he does not recognise in court the client of the station who was with him at the same time and interacted with him for a few minutes.
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.
Reika Hayami and the Uninvited Guest - S1-E5
Stupidity: A rather unnecessary part of the plot focuses on the convict trying to learn Reika's last name by ordering something on her behalf to read the name on the bill. That's absurd; he couldn't possibly know if the bill would just have the number of the cabin on it instead, or the production company's, and quite frankly for such a VIP (the cruise itself was a one-evening trip around the bay just so fans could spend time with their idol Reika, and he knew that) the ship wouldn't even charge anything for a drink brought to her room.
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.
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.
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.
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.
