Episode #3.7 - S3-E7
Stupidity: Humphrey picks the gun from its case using a pen to avoid leaving prints (since for some reason he's not wearing gloves), but then starts messing with the box with his bare hands without having it dusted for any print. Since the box was owned by only one person and hidden under his bed, any other print would have been important evidence. (00:11:40)
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 problems start when they try to get the man into the house without Jackie noticing. It's a bit odd that their first idea is to put him in Martins car, since that's the car they're going in. The best option would be to go through the front door and up the stairs since Jackie was going through the living room. As concerned as Martin is about Jackie finding out about the oil on the carpet, I don't think he would forget about the front door.
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: In the flashback, Emily's sister takes ill while playing her flute. In the next scene, the sister is sick in bed and Emily now has the morpher around her neck, but she is in the exact same outfit. If the sister was forced to pass the legacy on due to disease, it should have taken more than just a few minutes or even hours to learn that.
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.
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.