Episode #3.2 - S3-E2
Stupidity: Throughout the whole episode, no effort at all is made to find out where were the chocolates purchased, and the bottle of champagne that accompanied the box(es) is completely ignored, not even addressed in a throwaway line (such as that it's too common to trace, or whatnot). It's not directly part of the murder trick so it not even considered. The explanation of why the other incriminating box is still there is also quite ridiculous, since the murderer is left alone for enough time to do strike again with a very timely effort involving her leaving the only room of her bungalow (which involves her, the most known and conspicuous person on set, stalk her in plain view for an undisclosed amount of time), but could not find a few seconds to take a walk a few feet away and bury the box in the sand or bushes, flush the poisoned chocolates down a toilet, throw them at sea, just about anything. It's a bit of chocolate and cardboard, not exactly hard to dispose of that in a big set in the wild.
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.