The Ringer - S1-E1
Stupidity: As Link is watching Zelda from his tower, he's completely distracted and oblivious to the Moblins entering his room from the floor. They have the element of surprise and one sneaks up behind him. But rather than doing the smart thing and say, stabbing Link in the back or delivering a crushing blow from behind while Link is unaware of his presence, the Moblin simply just grabs him on the shoulder and puts his arms behind his back to hold him while the other Moblins go for the Triforce. Giving Link plenty of ability to kick one in the face, free himself and grab his weapon to fight them off. The Moblin should have just gone for the stealth kill, even if they aren't all that bright of an opponent. (00:02:10)
And All Through the House - S1-E2
Stupidity: The woman could've blamed her husband's death on the maniac, she didn't need to go outside to retrieve his body.
Double Sin - S2-E6
Stupidity: How in the world did Miss Lemon spend 2 days (and nights!) in the office searching for the keys but without looking on the console right next to the door? (00:40:25)