I recall seeing a Powerpuff Girls episode with a super-villain school. In one scene a teacher was next to a blackboard, and on the blackboard were the syllables "Mwa-ha-ha-ha". The students were reading this aloud in the most mechanical, non-super-villain-laugh manner possible.
Any idea which episode this would be? I suspect Mojo Jojo was the teacher, but I'm really not clear on that.