Stupidity: When Mac and Lindsay are reconstructing the stab wound found on their victim on their pig specimen, a couple of the weapons they use are long, thin knives; since they obviously don't match the circular wound, Mac at the very least would know those could be excluded by default.
Tag Team Trial: Part 2 - S1-E11
Stupidity: After Jaden summons EH Tempest, he also activates the field spell Skyscraper. This means that Tempest gains 1000 attack points (going from 2800 to 3800) when it attacks Dark Guardian (whose attack points are 3800). Since Dark Guardian can't be destroyed by battle, Jaden sacrifices a card to keep Tempest on the field. What was Jaden expecting? He already knew about Dark Guardian's effect, and there would've been no life point damage, so this attack wouldn't have benefited him in any way.