Episode #3.6 - S3-E6
Stupidity: The villain concocts a demented plan that puts him at absurd risk (if anything goes wrong, he is caught red-handed) and accomplishes nothing, since it does not make him and his accomplice unlikely suspects any more than anyone in the group. It does not make it look like it was an outsider doing it (no attempt is made to fake a robbery), nor a result of an accident. It makes obvious it was a murder (a knife plunged in the back!) but does not pin the guilt on anyone else in the team, which would have been really easy to do (plant the knife or any of the victim's belongings in anyone's tent) and with the two being in on the scheme without anyone suspecting they were connected to each other, it would have been easy to create an alibi for each other, but they don't have any. It should also be noted that there's no reason why the victim wouldn't have exposed the culprit's crime earlier to his fellow birders, since he knew he was going to harm the animals.
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.
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: 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.