Beware the Jabberwock, My Son - S4-E5
Stupidity: When the V'd Up Bull is about to attack, and then after the mutant sheep start attacking the group, suddenly Neuman forgets she can pop their heads with her power. She doesn't even try. Moments earlier, she was shown popping a V'd Up chicken to save her dad. When they are trapped in the barn with the sheep outside, and later when trying to escape before the sheep get sick, she doesn't even try to pop them.
When in Rome - S1-E3
Stupidity: A supposedly covert camera is placed in plain sight on a wall in a crowded room, with a prominent antenna showing, and a blinking blue light. (00:16:55)