Julia Gets Her Head Stuck in a Fence - S4-E8
Stupidity: Once he decided to finally saw into the banister, all Anthony had to do to get Julia's head out was to saw through one of the thin posts on either side of her head. Instead, he saws through an enormous section of the staircase, later causing the governor to accidentally fall through. If he didn't realise that, one of the other designers, all who were looking on, should have. It's a lot to presume they all would have been that careless just for the benefit of a sight gag.
Stupidity: Amazing how people on this show, like The Accused here and in the earlier episode "The Chef", are leaving their doors unlocked or no explanations given as to how the actual killer planted the evidence.