"Never Trust a Snake" is an episode of Ninjago: Masters of Spinjitzu that aired sometime in 2012. The theme of the episode is the repetition of mistakes, as none of the characters ever seem to freakin' learn from their screw-ups.
Zane has a prophesy that turns out to really have no meaning once viewers find out that Lloyd eventually becomes the Green Ninja. Later, while training, Wu asks the ninja the best way to defeat an enemy, and (SHOCKING!), the episode does not end twenty minutes early because one of them guessed the right answer. Lloyd leads the Fangpyre tribe to attack the Hypnobrai, but (again, SHOCKING!), it turns out that the Fangpyre tribe doesn't actually feel the need to be led by a naïve boring little kid and instead ally with the Hypnobrai. Lloyd then, learning absolutely nothing about how f*****g ungrateful snakes apparently are, rescues the cannibalistic Pythor (really, one of the darker characters of the series solely for that reason) from his tomb. Pythor takes ages to betray him, though, and they make fools out of themselves for a bit first. Eventually, they go take revenge on the Boarding School for Bad Boys*, which somehow meets Ninjago's education standards or something. Man, what a messed up place.
The ninja come there to free the school, and are involved in a pointless cameo by Kruncha and Nuckal. Another unsurprising turn of events occurs, and Pythor betrays Lloyd, but the ninja rescue him.
On Destiny's Bounty, Wu reads Lloyd some anti-snake propaganda and Lloyd falls asleep. The ninja then learn from Wu that the answer to his question at the beginning of the episode, about the best way to defeat an enemy, is to make them your friend. The ninja nod their heads in understanding, then neglect to try and make any other diplomatic resolutions for the rest of the series.
*Were the teachers at the school also bad, or were they good to? You never actually really see them: it's only the students that get involved and do anything good or evil...