The Secret of the Flame Tree - S6-E2
Stupidity: Sylvie Baptiste is a successful writer, and she published novels after her masterpiece. The idea that she'd lose all her fame and fortune if someone published an academic paper making a completely unsubstantiated claim (if Sylvie does not have an original manuscript of the novel, neither does Lizzie) about her novel being the work of her crazy sister, is simply absurd. A good copyright lawyer or even any decent PR agency would put the appropriate spin to the accusations easily, and since her sister is certainly not gonna sue her and Sylvie is her only possible tutor, all those people supposedly very well connected with the academia and industry and shrewd in marketing their work, overreact for nothing. Even better; Humphrey has not even exposed the killer (or that it is a murder at all), and the university announces already that they plan to give a posthumous PhD to the deceased, for the paper she hasn't finished, based on an accusation with no evidence.
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: 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.
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.