I don't really know about worst advice but worst class experience.
My friend and I took a class for a birthday table runner because we wanted to learn to machine applique. We were not allowed to cut our fabric ahead of time because of a couple of tricky pieces. We spent most of the class time cutting our pieces, sewed a few together and then the class was over. Never did get to the applique and haven't yet learned how. One of these days I will bite the bullet and just do it!