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.
Stupidity: The elaborate plan of the bad guy obviously involves driving Magnum increasingly crazy and create a rift between Higgins and him, but practically all the most damning episodes are outside the bad guy's control, and all the steps he takes are actually harmful towards him should have Magnum been in any way competent at his job. He couldn't have predicted that Magnum would paint the whole episode under the ridiculous light of "sumo wrestler and midget" instead of "big guy and small guy", he couldn't have predicted he'd stain himself, he'd try and force himself upon the girl, he wouldn't check the girl's background.
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.
Episode #3.2 - S3-E2
Stupidity: Throughout the whole episode, no effort at all is made to find out where were the chocolates purchased, and the bottle of champagne that accompanied the box(es) is completely ignored, not even addressed in a throwaway line (such as that it's too common to trace, or whatnot). It's not directly part of the murder trick so it not even considered. The explanation of why the other incriminating box is still there is also quite ridiculous, since the murderer is left alone for enough time to do strike again with a very timely effort involving her leaving the only room of her bungalow (which involves her, the most known and conspicuous person on set, stalk her in plain view for an undisclosed amount of time), but could not find a few seconds to take a walk a few feet away and bury the box in the sand or bushes, flush the poisoned chocolates down a toilet, throw them at sea, just about anything. It's a bit of chocolate and cardboard, not exactly hard to dispose of that in a big set in the wild.
Beware the Jabberwock, My Son - S4-E5
Stupidity: When the V'd Up Bull is about to attack, and then after the mutant sheep start attacking the group, suddenly Neuman forgets she can pop their heads with her power. She doesn't even try. Moments earlier, she was shown popping a V'd Up chicken to save her dad. When they are trapped in the barn with the sheep outside, and later when trying to escape before the sheep get sick, she doesn't even try to pop them.
Stupidity: This highly prepared organisation knows Dracula is coming, but comes armed with guns they're not prepared to use, and allow him to grab a weapon and shoot one of them. Then they give him a tablet but assign the most basic of passwords to the Wi-Fi, rather than disabling it entirely, or even just giving him a Kindle, given the only point of it was access to books. Plot-serving idiocy.
Stupidity: Andor and Luthen set a vehicle to drive and distract the security team. The vehicle is destroyed, and they immediately round the corner in full view of the team on a speeder and drive away, then blow up the vehicle. They could have driven away on any number of other streets without revealing that they weren't in the vehicle, keeping their escape secret. And if they blew it up before the team looked too closely they might have even been deemed to have been killed in the crash. (00:32:19 - 00:32:43)
Stupidity: Recurring issue when Ahsoka is fighting with dual sabers - there are often times in a duel when one saber is clashing with the other person, but she still has a hand free with her shoto that she could use to stab/slash and win the fight easily. Would make for shorter battles of course.
Free Family Portrait Studio - S8-E24
Stupidity: Dwight is seen walking out of the bathroom with the diaper in the direction of the main part of the office in which Angela sits. If he was to try to stay hidden (which was obviously the whole point), he would have gone through the exit of the annex, waited until she was distracted/or hide it for later and Angela never would have seen. This was done because the plot required for Angela to notice but Dwight could have easily kept it hidden. (00:10:40)
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.
Stupidity: When the bad guys are discussing the plans inside a private jet, the scene finishes and there's a shot of the plane in the sky. The plane immediately veers to the right, and spins almost 90°, as fighter jets do when they turn quickly. If there were three people inside, (one of whom was in the bathroom) they would have been thrown about the cabin quite violently. (00:11:25)
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: 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.
Stupidity: When Franklin and Tara are hooking up at the motel, it appears that sunlight is shining through the windows. Franklin is unaffected.
A Knight of the Seven Kingdoms - S8-E2
Stupidity: It is decided that those who cannot fight will hide in the crypt from the army of the dead, and Jon Snow suggests during the war council to put Bran in the crypt as well, "where it's safest." Jon knows full well that the Night King can resurrect the dead and add them to his army. Not only should he not suggest to hide Bran there, he should be telling everyone not to go anywhere near the crypt.
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)