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.
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.
Stupidity: The whole episode revolves around Adam bringing his girlfriend Emma to his parents' house to meet the family, but problems start to arise when Emma's sister begins texting Adam pictures of her in her underwear. Adam is extremely bothered by this and attempts to make sure Emma doesn't find out. When everyone is at the dinner table, Adam decides to leave his phone out on the table even whilst he continues to receive texts from Emma's sister and the family begin to comment on it. If Adam was so concerned about Emma finding out about the texts, he could have easily turned off his phone and put it back in his pocket, but instead decides to leave it out on the table and switched on, only drawing more suspicion from Emma. Even when the little girl arrives later and asks to play with his phone, Jonny reaches out to grab it, but all Adam does is take the phone off the table so Jonny can't reach it, and then immediately puts it back on the table again.
Suggested correction: Adam knows Jonny is joking with him, as he usually does. As for Katie taking his phone he didn't expect her to. They have all seen Adam's reaction when getting the messages and he is trying to cover something up. Doing anything to prevent anyone seeing his phone more would arouse suspicion. Personally I don't understand why he didn't just explain that Emma's sister has been sending him pictures and ask Emma to talk to his sister. He didn't do anything and loses someone he liked for no reason.
I was mostly referring to the scene where they are sitting at the dinner table BEFORE Katie arrives. Despite the fact that he kept receiving texts from Emma's sister and was growing more concerned and everyone begins to comment on it, all he does is put it back on the table, switched on, ready for another text to come through, which makes everyone further suspicious. There is no reason at all why he couldn't just turn off the phone, put it back in his pocket, and lowered suspicion.
Stupidity: Harold isn't sure whether to trust the flash drive Claire gives him, worrying connecting it to his laptop will send Samaritan all his personal connections. Somehow it doesn't occur to him to just use a completely disconnected / airgapped laptop, in a Faraday cage or similar like the one he kept Root in. If the flash drive had malicious code on it, nothing could be transmitted anyway.
Stupidity: An RAF officer is wearing a gold stripe on his shoulder boards instead of blue. On his lapels, he wore a corporal's insignia, and on his left pocket was an American colonel 's eagle. A soldier described as British was wearing American platoon sergeant stripes.
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.