The most influential class I took in undergrad was a class with undergrad/grad students. The graduate students had extra requirements of course. Experiencing the class with them, help me to understand what graduate school was like and motivated to work harder to keep up with them.
I believe the course had 2 different numbers for graduate students and undergraduate students.
I also learn what pedagogy meant after a PhD student was nice enough to tell me. Whenever I hear that world I think of her.
My first course in what is now my area (it wasn't even my major until much later) was a split level course.* I might never have dropped out of school if I had been allowed to skip straight to advanced coursework in more areas.
*This is not universal, but I consider "split level" to be a grad/undergrad enrolled course. "Cross listed" refers to a course that can count in more than one department/discipline, like a Latinas in Politics course that can count as Women's Studies, Ethnic Studies or PoliSci.