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.
Hanging with Axel: Part 2 - S3-E5
Stupidity: Hassleberry and Jesse sneak around Axel to free Syrus from the tree whilst Axel is distracted with the duel. They are trying to be discreet whilst Axel is distracted, but Hassleberry shouts "Hey, Sarge. Open up a can of Neos on this guy" to Jaden, even though Axel would definitely hear this. (00:06:38)