The Promise - S6-E13
Stupidity: Markham lets Ava call her uncle's phone, and she doesn't let on that Boyd answers instead. He later says "I don't trust anyone", but he didn't demand she put the call on speaker for transparency, he's quite happy (at the time) to just hear one side of the call.
Blade VS Buffy Summers - S12-E10
Stupidity: Blade fights several vampires for a minute or two, and then pile drives the Asian lady vampire into a table. Only then does he decide to throw a UV bomb up into the air to kill all the vampires in the room. He should have led with that! Wiped them all out from the start, and by not doing so, he gave plenty of vampires time to escape. (00:19:06)
He Lokomaika'i Ka Manu O Kaiona - S8-E20
Stupidity: After being stripped of their weapons while looking for terrorists in the jungle, Steve and Catherine meet up with Jerry, who has not had his gear taken, but didn't bring a gun because he's only a consultant. Some time later, after they open some coconuts using a small machete Jerry had on him, they hear gunshots, and go over to investigate, finding two dead hunters. They take a pistol of one of the hunters right before encountering the terrorists. So far, so good. But then to fight the terrorists, Steve has Catherine provide cover fire with the handgun while he attacks them unarmed, and hands Jerry a rock when he asks what he's supposed to do. Why doesn't one of them take the machete? You can still see it on Jerry's back.
Suggested correction: If you are not trained in fighting with a machete, doing so may well do more harm than good to you. Hard to believe that Superguy Steve isn't, though.