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.
Stupidity: Hard to swallow that a group of vastly experienced policemen smart enough to figure other tricks out (such as the abnormal position of the gun) would be in any doubt about the victim being left or right handed when she is still wearing their watch on the right wrist - and attention is called by Poirot upon that particular detail right from the start.
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)