Two undergrads taking the class because it just sounded neat, and the professor made things interesting. 10 graduate students who were teachers taking the class for the increased salary. An evening seminar of three hours per week after work where we examined as much as possible over how people did math through the ages and through time.
You could tell these grads were bored. But, fine, dealing with kids all day, then having to learn something new that isn't likely to ever be more than a trivia point in a lecture might not be a great thing.
Then... We get to a point where it looks like Archimedes came this close to discovering integral calculus. [insert appropriate WOW! meme here]
A couple of grunts from the grads. Me and the other undergrad were gobsmacked.
There were other examples, but that's the one that always sticks in my head. It's also what contributed to me not being very impressed by some of the grade school teachers I met as my kids were enrolled.